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Old 09-06-2018, 08:23 PM   #1
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Amp Hours or State of Charge

Ok Iím having trouble with AH vs State of Charge.

I have a 150 watt solar panel and two 235AH 6V golf cart batteries in Series. I also have a 1500W modified sine inverter.

All I want from my batteries is to watch a few hours of tv at night.

If I start with a good charge on my batteries, and watch TV for a few hours with a total draw of 6A, my battery voltage will drop to about 12.0 and sometimes 11.9V but according to my battery monitor, I have only used 20AH.

I know that State of Charge is measured with No Load and after a few hours of the battery at rest. The 11.9V that I read is with a load of my inverter, TV and Directv receiver/dish (G3 Carryout).

Iíve said all that to say this:

What should I be going by? 11.9V or the 20AH? According to SOC chart, 11.9V is 60% discharged but according to 20AH, my batteries are only about 15% discharged. I donít want to take the batteries below 50% cuz they are wet cell.

Some people have told me to go by the AHs and disregard the voltage BUT if I do that, eventually the voltage will get so low that the batteries wonít power anything.

Please help me understand this and which method to go by.

Thanks!
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Old 09-06-2018, 08:51 PM   #2
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I would get a hydrometer and measure the specific gravity of the electrolyte of each of the cells. That is probably the most accurate measure of charge and the health of the batteries. When is the last time the batteries were equalized?
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Old 09-06-2018, 11:20 PM   #3
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First, while solar panels can recharge batteries, I would not rely on solar panels to maintain batteries in top shape. To determine if a battery is good or bad, it needs to be fully charged and load tested. Load test it any way you like. However if a battery cannot produce either a lot of current all at once or a little current over a very long period of time, or both, it is no good.

Provided open circuit voltage is at least 12.7 volts after removing plate surface charges, a battery might be considered fully charged. However an equalization charge might result a more complete charge when specific gravity readings measured with a hydrometer between cells vary by more than 30 points. I really don't care much about specific gravities. Specific gravities are interesting and sometimes useful. I really care more about open circuit battery voltage at full charge after plate surface charges have been dissipated. And more importantly, that actual current load a battery or series of battery's can sustain over time. I have seen plenty of batteries with good hydrometer readings as flat as a tire.

Battery aficionados might like measuring amp hours in, amp hours out, and all that stuff. I don't get wrapped around the axle about all that stuff. I make sure my batteries are chemically good, and charge them with regard to their ability to accept a charge over the time and current available to charge them. And use them not working about all that amp hour stuff. I know they will power my roof air conditioner on inverter for about 1.5 hours and I know they will power my microwave in the morning to make coffee with plenty of power left over when not having the capacity to power the AC or microwave to run the lights, water pump, or refrigerator for a couple of days. And I never mind how discharged they get while some folks would rather freeze to death than run a battery below 50-percent state of charge. I'll recharge them as soon as possible to run them down again as much as I like.

Even after my batteries are too low to power the AC or microwave, they will power my 40-inch and 24-inch flat screen 12V TV's for hours. Maybe the problem is relying on an amp hour meter rather than charging batteries as full as possible and using them "seat of the pants" based on the chemical ability they have to produce current flow. At 11.9 volts the batteries are virtually dead flat. I am not a fan of 6V batteries as I have found them more difficult to charge as they seem to be more resistant to accepting a charge, especially as they age.
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Old 09-06-2018, 11:37 PM   #4
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AC for an hour and a half?
What batteries, inverter, and AC.

I want my inverter to absorb inrush so I can run a smaller generator (yet to be purchased). Not there yet. Even on shore power I was blowing the output breaker on the inverter. It's a 3k aims with built in transfer switch.
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Old 09-07-2018, 08:44 AM   #5
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Amp Hours or State of Charge

Iím still not convinced that I have an accurate answer guys but thanks for the info. Itís well known not to discharge flooded batteries past 50% because it decreases their life span so the ďseat of your pants methodĒ is not going to work for me. 99% of RVers cannot run their AC on batteries so Iím not sure what kind of set up you have Split but it must be much better than mine. Itís 11.9 with the inverter and TV on. It jumps up to 12.2 or so with all that stuff off. All I want to know is if I should be going by AH or the Voltage to determine how low my batteries are. Can someone with a Battery Monitor chime in? I also donít want to break out a hydrometer every time i run my TV and Inverter. Thatís why I bought a Battery Monitor.
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Old 09-07-2018, 09:09 AM   #6
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We've never used anything other than our Magnum ME-RC panel readout of VOLTS, while using the Inverter. Yes, the Volts reading is a 'real now' type of number, while under load, but I don't worry much about whether it's the 'exact' number I have to use or not, but it's the number I use to set my system, and have a comfortable 'normal' relationship with....

for example.. at full charge, while plugged in or the generator running, the VOLTS might show 13.5 or 14 or so. I don't really care whether that's exactly accurate, but it's what I 'normally' see at full charge.
I do know, though, that once I unplug, or turn the generator/charger off, the Volts are no longer going to stay at that Voltage - they are going to drop to 12.8 or so, with very little 'load', such as the typical items that are usually 'on' in the coach, such as lights, or floor fans, or the tv and sat receiver. I view 12.8 or so to be 'fully charged' for the Inverter.

Now, as soon as I add any device of substantial 'amperage', such as the microwave, I will immediately see the VOLTS drop suddenly to 11.8, then 11.6, then 10.9...the longer the microwave runs, the lower the number.... BUT, I also know that this Volts reading is a simple 'indicator' of the power the microwave is drawing, but NOT really the 'status' of the house battery levels themselves, as once the microwave goes off, within a few seconds the VOLTS reading goes back to a 'normal' reading level of 12.6, or 12.2, for example.

I only get concerned about the VOLTS reading when it falls below 11.5 or so, as my experience, especially early on, taught me that it's not only about how 'much' you should let your house batteries discharge, but whether there is enough 'Volts' to start the generator when you need it to RECHARGE the house batteries!

While you may not have an AGS(auto generator starter), like I do, my settings are similar to how you would want to view your 'manual' operation:

- AGS setting 'Enabled'(on/ready)
- 'Shore Power' button setting to 40amps(the max amps the Charger is provided)
- AGS 'Volts' setting at 12.0 volts(when the Gen is requested to start)
- AGS 'Run Time' setting at 30 minutes(how long the Gen will run)

also,
- 'LBCO' setting to 11.5(will cut off the Inverter if the battery levels fall below these volts)


for me, I use 12.0 volts because we've found that it is a level that combines both a good level for the recharging to start, and not too often that 'regular' inverter usage doesn't require the generator to run too 'often', and doesn't run too 'long' when it does. Each person has a different usage style and will adjust this to their own comfort level.

The LBCO is a terrific 'safety' feature that limits the Inverter's draw by cutting it off if the levels of the batteries fall too low, and the generator could not be cranked... I've found that while you 'might' could still crank the generator at some lower numbers, 11.5 is a good level, just in case, to preserve more level. Normally, the generator cranks just fine, but if you find you forget to check the fuel level, and overnight it uses enough to not be able to crank, your Inverter might continue using enough amps that the batteries fall too low to crank the generator when you DO get the fuel, etc.


Nobody does any of this the same, we all camp and use our resources differently. I think you might not want to be so concerned about 'which' unit of measure is the most important, or even which is the most accurate, but just rely on one to draw future experience in making those decisions. SOC is great, but I don't have that measure, and don't think that it's really any detraction to great camping and travel. Use VOlts and you'll be just as fine.
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Old 09-07-2018, 09:16 AM   #7
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i do not have a battery monitor so i have no actual experience. i will get one but i suspect they are not as accurate a device as the marketing literature makes them out to be. there are just too many variables involved.

the amp hours measurements in and out are probably pretty accurate as the current has to flow through the shunt and an accurate measurement can be taken. but what does taking 20 amp hours really mean? what you want to know is what is still in the battery. how do you specify the amp hour capacity of the battery? you would need to know what the capacity is in order to determine how much is left after you take 20 amp hours out. and we know that batteries degrade over time so the capacity will degrade over time such that taking 20 amp hours out of a degraded battery will leave less capacity remaining as compared to taking the same out of a new battery. add in the possibility of a battery disconnect switch which removes power to the monitor and you've just lost the data stored in the monitor memory. or current that flows internal to the battery as it self discharges. monitors get around this by resetting the battery capacity when they get charged and the monitor senses that they are fully charged. but again all the monitor can really know is that they are fully charged (maximum amp hours are in the battery), but what is that amount in amp-hours?

state of charge seems to vary based upon whether the battery is active (charging or discharging) or at rest.

bottom line, i think they are still a good device and supply some useful information. but i suspect that information still needs to be filtered through your past experiences to give you a relative feeling of state of the batteries. the best they can do is to provide a relative reading that needs to be interpreted.

these are just my thoughts. i may be wrong and i may be missing something.
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Old 09-07-2018, 09:23 AM   #8
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Thanks. Those are good general guidelines. My LBCO is set to 11.8 currently, I will probably lower that setting. I usually run my generator a couple hours each day to top off the charge that my solar canít provide since itís only a 150W panel. The explanation you provided for the different voltages and loads is useful too. I could never run my microwave or coffee maker because my inverter shutdown under that type of load (with LBCO set to 11.8).
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Old 09-07-2018, 09:27 AM   #9
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true, the LBCO is a good safety 'backup' feature, BUT, it can be also a little annoying if you forget, and decide to run your microwave too long - the LBCO will cut off the Inverter once the level has been breached, and that can happen pretty quickly if you don't either keep an eye on the microwave, or manually turn down the LBCO during the use of the microwave.
the most annoying part is the fact that the satellite receiver has to be 'reset' and reacquire the satellites after it loses power - annoying when it happens, though, thankfully, not too often.
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Old 09-07-2018, 09:28 AM   #10
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Email the battery monitor mfg and ask them.Ö
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Old 09-07-2018, 09:32 AM   #11
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Iím still not convinced that I have an accurate answer guys but thanks for the info. Itís well known not to discharge flooded batteries past 50% because it decreases their life span so the ďseat of your pants methodĒ is not going to work for me. 99% of RVers cannot run their AC on batteries so Iím not sure what kind of set up you have Split but it must be much better than mine. Itís 11.9 with the inverter and TV on. It jumps up to 12.2 or so with all that stuff off. All I want to know is if I should be going by AH or the Voltage to determine how low my batteries are. Can someone with a Battery Monitor chime in? I also donít want to break out a hydrometer every time i run my TV and Inverter. Thatís why I bought a Battery Monitor.
I have just 12 volt batteries , three hooked up and can watch just tv thought a 800 watt inverter for 5 days without issues or before recharging . tv's do not take much amps . you may have other big draws taking the batteries down . how old are your batteries , what else is being powered off the batteries , how big is your inverter, etc .
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Old 09-07-2018, 09:57 AM   #12
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Batteries are 2 months old. All connecting cables are as short as possible and the correct gauge. The inverter is 1500 Watts. Nothing else is being powered at the time except for the inverter, tv, Directv receiver and the dish itself which requires power. The total draw is 6A which isnít much at all. Fridge is on gas.
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Old 09-07-2018, 10:15 AM   #13
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Batteries are 2 months old. All connecting cables are as short as possible and the correct gauge. The inverter is 1500 Watts. Nothing else is being powered at the time except for the inverter, tv, Directv receiver and the dish itself which requires power. The total draw is 6A which isnít much at all. Fridge is on gas.
6 amps is quite a bit , the direct receiver takes a lot as does the dish it;s self . a 1500 watt inverter is also a power drain . i agree you should be able to get more then one night viewing . the fridge even on propane still uses 12v power , along with the co detector , lights, etc . It's quite possible your monitor is wrong . only way to know is to test batteries with a separate meter and see if it agrees with your monitor . everybody should have a meter anyways that tests voltage AC & DC along with continuity .
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Old 09-07-2018, 02:34 PM   #14
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Batteries are 2 months old. All connecting cables are as short as possible and the correct gauge. The inverter is 1500 Watts. Nothing else is being powered at the time except for the inverter, tv, Directv receiver and the dish itself which requires power. The total draw is 6A which isnít much at all. Fridge is on gas.
In a previous response, you mentioned using your generator to complete your charging. You are correct that a 150 watt panel can't keep up with the roughly 100 Amp Hours (or more) your battery bank can deliver. This is wise, because undercharging leads to sulfation.

Your 6 amp draw, totaling roughly 20 AH, should be a piece of cake for your battery bank. For comparison, I have a single group 24 12-volt battery (now on its 5th season) and I run an electric blanket (360 watts thru a 400 watt inverter) for 30 minutes. That's 15 AH all by itself. Then, because it's cool in CO in the mountains, my furnace - 5 amps - may run on a 50% duty cycle for 8 hours or so. That's another 20 AH. What with the water pump, spark ignition for the water heater and fridge, and the relentless draw of the CO/propane detector, I'm pushing 40 AH out of a battery that should deliver 35 on a good day. Yet all is well...for years...with just a single 100 watt panel in the reliable CO sunshine.

Where am I headed? You have another problem if your battery bank is dropping below 12 volts. It could be a bad meter, but that's unlikely. A cheap multi-meter can answer that question in a hurry. The hygrometer can perform a real test/diagnosis, but that's about as convenient as surgery on a regular basis. Definitely get one and do the tests, but for day-in-day-out use, verify that your volt meter isn't on the fritz.

Someone mentioned cell balancing - Battery University is a good source.

Others mentioned a mystery draw...sounds possible. If you exhaust your other options, begin the hard work of tracing a phantom power draw.

Another possibility. Is there any chance that your converter/charge isn't functioning as it should? You're relying on the generator-driven "bulk" charge (constant current) and/or topping charge that your converter/charger should deliver. If you have, or can buy, a decent home garage charger...not just a little float charger, but the kind that can quickly charge and/or jump start a car...charge with that and see what happens. Maybe your WFCO charger has failed...or close to it...maybe just delivering a float charge. Plug the garage charger straight into the genny, and do not connect the genny to the shore power on the converter. The external charger might reveal the problem. In fact, most have meters, and if the meter pegs at 10 to 15 amps of charge for an extended period (15 minutes or more), you'll know right away that your battery bank is low and craving charge. The garage charger should peak briefly at 10 to 15 amps then cut back to 4 or 5 amps topping charge if the battery is pretty close to full.

Your comments demonstrate that you are well-informed, so I offer this with tongue in cheek. Check your math with this calculator. Bear in mind that ALL LOADS are 12 volt loads when on battery...even if they are 120 volt devices. 100 watts at 120 volts = .83 amps, but at 12 volts, it's 8.3 amps. That's why my 360 watt electric blanket draws almost 30 amps per hour. If your 120 volt TV is rated at 200 watts at 120 volts, it's pulling nearly 17 amps at 12 volts all by itself. Add in the satellite gear, and suddenly a couple hours of TV is starting to use serious power.

Don't forget that even LED lights and all the other "tiny" uses add up in a hurry. Run your numbers on everything just to be sure. My little PUP furnace pulls 5 amps. A central, ducted, forced-air system can pull double that. Your pump pulls roughly 5 amps, and, and, and. And, if in reality, your solar is really all that's charging your battery bank, you might be running a big deficit.

But with all that said, and with a battery bank capable of delivering 100 to 120 AH, you should not be dropping to 11.9 volts. Something's up.

And, while you're at it, I'd presume your batteries may not have been fully charged recently, and they may have sulfation and need remedial conditioning. Battery University has good info on diagnosing and addressing sulfation.

Good luck.
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Old 09-07-2018, 03:20 PM   #15
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Amp Hours or State of Charge

Jim, thanks for the detailed response. THat makes sense. Here are a few follow up comments:
1. After the 20AH is drawn and I start my gen in the AM, it supplies about 18 amps and is in bulk charge state. That tapers down over the 3 hour charge period to about 5.0 amps. The last bit of charge takes forever and I hear thatís typical of the gens.
2. I have measured with a meter at the batteries and that concurs with battery monitor bolts reading.
3. My TV and receiver are 120 volts. My inverter has a Meter on it too and it says the load is 3% (1500 watt inverter at 3% Load is about right at 45 watts or about 4.5-5 amps if my math is right). So the battery monitor reading 6A draw with inverter overhead of 0.5A sounds correct to me.
5. If I add up the amps of the appliances in use and go by your calculations here is what I get: TV (100w/12) = 8.3A. Directv receiver is (24W/12) = 2A, carry out G3 (30W/12) = 2.5 A. All together thatís almost 13AH as opposed to the 6AH my Monitor is showing. Maybe itís pulling more current than the monitor is displaying? But even if itís pulling 13 AH over 3 hours thatís only 39AH total.
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Old 09-07-2018, 03:23 PM   #16
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How do I measure the current being drawn right at the battery. Can I use one of those clamp-on meters and just attach it to he positive lead that goes from the batteries to the inverter? Also I did not mention that I use a 30A to 20A plug converter and my inverter power goes into that plug adapter. Now, I do know about the current loop that normally causes so I installed a 120v switch to disconnect the battery charger when the inverter is running.
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Old 09-07-2018, 04:14 PM   #17
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Jim, thanks for the detailed response. THat makes sense. Here are a few follow up comments:
1. After the 20AH is drawn and I start my gen in the AM, it supplies about 18 amps and is in bulk charge state. That tapers down over the 3 hour charge period to about 5.0 amps. The last bit of charge takes forever and I hear that’s typical of the gens.
2. I have measured with a meter at the batteries and that concurs with battery monitor bolts reading.
3. My TV and receiver are 120 volts. My inverter has a Meter on it too and it says the load is 3% (1500 watt inverter at 3% Load is about right at 45 watts or about 4.5-5 amps if my math is right). So the battery monitor reading 6A draw with inverter overhead of 0.5A sounds correct to me.
5. If I add up the amps of the appliances in use and go by your calculations here is what I get: TV (100w/12) = 8.3A. Directv receiver is (24W/12) = 2A, carry out G3 (30W/12) = 2.5 A. All together that’s almost 13AH as opposed to the 6AH my Monitor is showing. Maybe it’s pulling more current than the monitor is displaying? But even if it’s pulling 13 AH over 3 hours that’s only 39AH total.
As it was clear previously, you have your head around this pretty well.

It sounds as if the converter/charger is working fine. Don't rule it out entirely, but 18 amps is really delivering.
If your entertainment consumes 40AH, and your generator/converter pumps in, in round numbers, 30AH in 3 hours, what of the solar?

A crude estimate of a 150 watt panel's capacity is 12.5 amps. More realistically, let's call it 10 amps. Now, let's presume 30% efficiency over the course of the day...an average of 3 amps x 12 hours. That's another 36 AH. So, at minimum, you are pushing 60 to 70 AH back into the battery.

I see you live in Tampa. If you're camping in FL, my estimates on solar are conservative to a fault. But let's stay conservative and assume you are REcharging a battery that leaves home full by replacing up to 70 AH/day with combined generator and solar charging.

Where does that leave you? How are you discharging down to 11.9 volts?
Perhaps you're not.
Or perhaps you have a phantom power hog.
Or perhaps there's something wrong with those expensive new batteries.
Or perhaps "somebody's leaving the lights on."

Maybe something here can give you accurate readings and peace of mind...or it will reveal that, even after you turn everything off, you have a steady power drain you weren't aware of. This one might just be the ticket. Just remember that the CO/Propane alarm demands a relentless trickle, but never so much as to cause your symptoms.

I confess that I hate these mystery problems. It's easy to get trapped in a logic loop that conceals the answer. I wish you luck. But you already have a good grasp of the variables, so please share when you figure it out.
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Old 09-07-2018, 05:12 PM   #18
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Rjpetr65---

Out of curiosity, what kind of battery monitor are you using. Is it one of the lesser expensive units sold everywhere from Amazon to e-bay or is it one that costs considerably more but includes in it's "brain" the ability to take into account what is happening in your battery under both discharge and charging currents (aka Peukert's Equation)?

Are you relying solely on Solar and Generator for recharging batteries or do you have the opportunity to plug into shore power every few days?

Unless you battery monitor is indicating a TRUE state of charge you may be suffering from a slow decline in state of charge. Solar may not be enough to charge all through the Bulk, and absorbtion cycle. You may be relying on your battery monitor to shut off the generator because it isn't compensating for charging inefficiencies and indicates 100% charge long before you actually reached that point.

If you are using a battery monitor such as a Victron, make sure it is set up correctly for battery bank capacity, Peukert Factor, Tail Current, etc, to insure accurate readings.

As far as using voltage readings or hydrometer to check battery health, make sure the battery has not been under load or charging for at least overnight. These readings are not accurate unless the electrolyte has settled down after charge/discharge.


After all this, give some consideration to just enjoying an evening of tv and don't look at your monitor until the next morning after you've had a good sleep. You might be surprised at how much the voltage will climb from during use to 8-10 hours later.
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Old 09-07-2018, 05:32 PM   #19
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Iíve lived exclusively on my solar for the past month. I monitor both the amps in and out,as well as the voltage, daily.

The voltage is the most reliable indicator of your charge status. There are several online charts that will indicate your percent of discharge based on the current load and the voltage at that load. These tend to be most accurate at low loads.

Something is absolutely wrong in your system. Most likely, Either one or both of your batteries are bad, or your charger is not working correctly. What is your voltage right after charging? It should be in the 12.8 range or higher with minimal load. How much battery capacity do you have at 12 volts? Two batteries is typically around 200 amps and your40amp load should draw you down about 20%. If your voltage is not starting at 12.7 or 12.8 with minimal load, your charging system is not working correctly or the battery is bad.

Fully charge your batteries using a different charger, then test them with a load tester. Any battery shop has one. These apply a heavy amperage load. A good battery will maintain good voltage during the test. A bad battery will drop the voltage quickly.

If that passes, check the voltage being put out during charging. Bulk charge should be at least 14.4, depending on the temp.

Iím a power hog. But I have 800 amps of battery and 860 watts of solar. I burn about 170 amps a night. my voltage willtypically drop to about 12.5 by morning.

My 2000 watt inverter actually has about a 5 amp overhead. While watching Tv at night, dish network, with a few lights, I am burning about 15-20 amps an hour. I burn about 60 amps each night with my cpap.

You are starting with less than a full charge. My system tapers to about 4-5 amps when full. However, it is 4 times bigger. Your batteries are not full until it tapers to 1 or 2 amps.

Finally, you may have other battery drains beside the inverter. My motorhome uses about 4 amps an hour with nothing obviously on. Sounds minimal, but thatís a whopping 96 amps a day. The fridge, alarms, etc.

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Old 09-07-2018, 05:45 PM   #20
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Notwithstanding all the disinformation on this post, and if your monitor is a decent one, voltage tells you NOTHING when the system is in use, it is useless. Having a monitor is the important thing, if it is a good one, either trimetric or Victron.

Just looking at your data, I would suspect whether you batteries are really capable of the 235 AH claimed, total being 235 at 12 volts.

If you want to know find a way to draw exactly 12 amps ish and record the time it takes to get to 10.8 volts, and if the is less than 20 hours or close, then you batteries are not working at rated capacity. When you get to 10.8 volts, charge them back up ALL the way, on shore power. Multiple the number of hours by 12 and that is your capacity, adjust you monitor to this new AH figure.

Doing this process several time may restore some of the capacity.

Yes a lot of work, but the only way to really determine what is going on.

Load testing is fine for starting batteries but useless for deep cycle.

Anyone telling you different is not familiar with lead acid battery chemistry.
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