How "new" should a battery in parallel be?

[gordonsick]

Originally Posted by HDU

All you are doing by placing batteries in parallel is averaging the output of two batteries. There is nothing wrong with using two batteries of different ages in parallel. A parallel circuit does not care, or know if the batteries are the same age. Go for it and have a good time.

I'm inclined to agree. It makes sense to have the batteries of the same chemistry (all flooded or all AGM), but the charge and discharge current should roughly balance out if they are of the same nominal voltage and in parallel.

The basic principle is that the small battery delivers current proportional to its capacity and the large one delivers current proportional to its capacity. If one gets ahead of the other in the discharge cycle, it drops to a lower voltage and the other delivers more than its share of the proportion until the two are balanced. As they both discharge in this manner, they tend to have the same voltages as they go down, since voltage depends on state of charge.

Now, an old and a new battery will have slightly different chemistry, so the proportions might not be perfectly aligned, which could present a problem, in theory. Fortunately, H Giess has actually tested this at
http://neuralfibre.com/paul/wp-conte...we-now-sin.pdf

The motivation for his test is telecommunications sites, which need periodic expansions of battery capacity to accommodate larger networks. The question is whether it is necessary to swap out all the older batteries when the new batteries are added. The answer is that this causes some variations from the proportional output that I mention above, but not enough to significantly reduce the life of the overall system. And, he was cycling these batteries pretty hard – he discharged them at the C3 rate, which is 30% of the amp-hour capacity. I do note that his test focussed on a massive mismatch in the amp-hour capacity of the batteries, rather than a mismatch in ages. The chemistry had some mismatch, since they had different C_3 to C_10 ratios. And the batteries didn't deliver current through the whole cycle in proportion to their stated capacity.

He noted that the larger capacity battery could have a greater capacity loss in situations of moderate discharge, rather than the strong discharge rate he tested. His Summary concludes that the loss is minimal: "However if these shallow discharges are infrequent (≈50 to 100/year) then the ultimate battery life will be governed mostly by calendar or elapsed service time considerations and not by exhaustion of its cycle life capital."

When I look at this on the internet, I note that (almost) 100% of websites hosted by battery manufacturers and battery dealers say that parallel mixing old and new or batteries of different amp-hour capacity (but same nominal voltage) is a really bad idea. Of course, it is a bad idea for them, but what about the user? That is, suppose the damage done by this is a 10% reduction in overall battery life and the user needs to replace 1 out of two batteries. Does it make sense to buy two batteries just to capture the extra 10% of life?

BTW, the author's affiliation is stated as Oerlikon Stationary Batteries in Switzerland. The company is a major industrial company, and is actually a battery manufacturer, so my 100% claim above is a bit overstated. Apparently, he is not grinding the axe that he would be expected to grind, given his employment.

–Gordon

[babock]

Originally Posted by carlsbad

I’ll never go back to lead acid. AGM or lithium. Yes, you pay more.

Likewise, but there are a lot of people that buy some flooded lead acid and think they are maintenance free if the top of the battery is flat.

[Palinduff]

When I decided to go with two batteries, my research told me use similar (or same age) and to fully charge them seperately before connecting and then always charge them together; that is positive on one battery and the negative on the other. That way they are always charged an equal (or close) amount. If you go with an old and then use a new one for the 2nd, the old battery will wear out before the newer battery, and that could cause some charging issues (I believe?).
I had a 2 year old 24 series, but decided on 2 27s. I probably should have gone with 2 6 volts, but with my previous TV, weight was becoming an issue.

[ependydad]

Originally Posted by Alleyantique

Question along the same line. I have noticed that no one ever recommends sealed maintenance free batteries. Is there a reason other than cost?

Oh, I recommend lithium drop-ins - but you usually need a defibrillator when the topic of cost ($800-$1,000/battery for 100amp hours) comes up!

I love my bank of lithiums and won't soon go back.

[Sealyn]

Originally Posted by HDU

All you are doing by placing batteries in parallel is averaging the output of two batteries. There is nothing wrong with using two batteries of different ages in parallel. A parallel circuit does not care, or know if the batteries are the same age. Go for it and have a good time.

Wrong- All batteries have internal resistance, different types and sizes and ages will have different resistance. The problem is they will charge at different rates so you may not fully charge one. This will affect total amp hours available.
Resistance difference is a critical factor in parallel circuits.

[Jonol]

Quote:
Originally Posted by HDU
All you are doing by placing batteries in parallel is averaging the output of two batteries. There is nothing wrong with using two batteries of different ages in parallel. A parallel circuit does not care, or know if the batteries are the same age. Go for it and have a good time.

Wrong- All batteries have internal resistance, different types and sizes and ages will have different resistance. The problem is they will charge at different rates so you may not fully charge one. This will affect total amp hours available.
Resistance difference is a critical factor in parallel circuits.

Wrong Guy`s!!
The charger will charge the 2 battery`s full charge but , the smaller one will be charge over stress charge and the charger will fully charge the bigges battery just find, then it will stop charging.
So try to use the same capacity and size battery and you will be find.

[gordonsick]

The Geiss paper that I cited earlier points to a paper "Operational characteristics of VRLA batteries configured in parallel strings" presented in 1998 at an IEEE Conference on Telecommunications Energy. Recall that AGM batteries are VRLA batteries, so the results with regular lead acid batteries might differ, but I doubt that. One of the authors (Cole) is at GNB Technologies, which manufactures Exide batteries.

You can see the paper at https://www.researchgate.net/publica...rallel_strings
or at
Operational characteristics of VRLA batteries configured in parallel strings - IEEE Conference Publication

Both of these sources require payment to download the article, but if you or a relative/close friend is a student or faculty member at a university, electronic access is probably free through the library.

Here is the abstract of the paper:
"It has become more common in telecommunications and other standby power applications to initially install the minimum size battery required and to subsequently install additional battery capacity as service loads and power demands increase at local sites. Many installations even provide planned unused space in the installation in anticipation for additional switches, rectifiers and batteries to accommodate future growth in a local area. Typically battery capacity is increased by paralleling one or more new battery strings with the battery already installed. Theoretically, this approach should be acceptable provided that certain precautions are observed. The purpose of this paper is to provide detailed data on the characteristics of batteries, VRLA designs in particular, operated in parallel configurations."

The tested parallel battery systems of different sizes, different initial states of charge, different cable lengths to simulate different internal resistances and came to the conclusion that all systems worked well and as expected. The small battery alongside the large battery produced less power, basically in proportion to its smaller size, but when both were recharged in parallel, they came back to the same state of charge. Starting with one battery being somewhat discharged, they both provided power and both sides came to a full charge when recharged. And changing the cable resistance by a factor of four (to simulate greater internal resistance) also did not change the results.

Note that all their batteries had similar fundamental chemistry and nominal voltage. So, we can safely make parallel circuits of different capacity, states of charge and age, so long as the batteries have the same nominal voltage (6v or 12v) and the same chemistry (AGM vs flooded). There could be issues with chemistry changing across brands.

–Gordon

[sportbikenut]

Originally Posted by carlsbad

Four months is new! Charge the “old” one. Charge the new one. Hook them up.

I agree. But also question, what is the definition of "New"?? I have looked at the batteries on the racks at multiple Costco's, Walmarts, and Sam's Club's recently and most of the batteries are date stamped 04/17. So, are these batteries new or 8 months old? They are certainly discharged. Would these batteries not be equivalent to a "new" battery on a trailer bought in 2017 and only used once or twice?

[pjnlorrie]

Former automotive/recreational battery distributor commenting-It can be difficult to nail down how close to "new condition" a battery is by age alone. As someone noted, a heavily used or abused battery may have a performance curve like a much older battery. That said, probabilities are 4 months is not going to be significant.
Sealyn brought up the aspect that will most often lead to problems; different battery types (flooded lead acid, AGMs, deep cycle) have different nominal voltages and internal resistances. Different battery brands, due to plate formulations, have different nominal voltages and internal resistances. The problem is that batteries with different chemistries/compositions with have different discharge/recharge curves. There are differences even in battery cells of the exact same type. This is why more sophisticated battery packs and their chargers have a cell balancing function. And as Sealyn stated, mixed batteries can often result in one battery being constantly over or under charged. Under charged = sulfation, short life, less than full capacity output. Overcharging = acid depletion (boil over-who wants acid sprayed all over in their battery compartment?), plate material loss, battery damage.
Specific data on battery characteristics is usually not available as each company uses proprietary formulas and construction. This makes matching batteries difficult. Also, battery manufacturers change their designs and plate chemistry from time to time. An Exide Gp24DC built this year may be quite different from one built the previous year.
Best practice, in an RV, is to install matched batteries by brand, type, and size. This will provide the best long term experience. Mixing batteries is totally possible, but extra monitoring is recommended.

[garbonz]

Funny that nobody asked what the OP was actually using the batteries for. Many use their batteries for occasional overnight part discharge, which a single battery will be bad at. Or they may be actually boondocking which is a different and more critical situation.

Having said that, the likelihood that adding an identical battery to an existing string of similar age would cause a problem is remote.

Hey, the OEM BATTERIES ARE JUNK anyway so what the worse that could happen.

[pgandw]

Originally Posted by garbonz

Hey, the OEM BATTERIES ARE JUNK anyway so what the worse that could happen.

I am the example for "what is the worst that could happen?"

We wanted our new A-frame to dry camp for up to 4 nights in the Colorado Rockies late fall and early spring (no intentional camping in snow).

Dealer agreed to install 2nd Group 24 Interstate Marine/Deep Cycle in a single large box. I was happy, and didn't pay the batteries much attention. I knew enough to disconnect the batteries when the camper was stored in the garage, and added water in the spring.

A year and a half after buying the trailer, we towed from Colorado Springs to Lincoln to visit family and watch football. The battery box cover blew off during the drive. Upon arrival, I put a temporary piece of plastic to protect something dropping and shorting the terminals. We had shore power, and everything worked fine.

When we got home, I measured the voltage, and found the batteries down around 12V. Plugged the camper in to recharge before disconnecting. After a few days, the voltage had dropped to 11.0V. Until I could get around to pulling the A-frame outside and popping the roof up to troubleshoot, I disconnected the batteries from the camper (still in parallel) and put my Sears 10 amp charger on.

The Sears charger promptly popped it's automatic circuit breaker every time I tried it. I figured it was overloaded, and disconnected the batteries from each other. I tried it first on one battery, and then the other, and the circuit breaker would trip after a few seconds on one battery, and immediately on the other. On the battery that took longer, I switched the charger to the 2 amp setting, and the charger held. After a few hours, I switched it back to 10 amps, and it continued charging. After a couple of days, it was charged. But I could never get the other battery to accept more than a few seconds charge. I forgot about things for a couple of weeks.

I got the camper outside, and popped it up, and found the converter 30 amp battery fuse blown. Replaced the fuse. Moved the camper back in, and popped it down. I then went to hook up the batteries to let the converter (WFCO 8735) charge them.

I forgot the batteries were at uneven charges. As soon as I hooked them in parallel, I had an amazing electrical fire in the battery box. The parallel cables glowed cherry red, and both batteries started boiling and/or gassing. The hydrogen gas was flaring from the vent caps, and the low blue flames engulfed the parallel cables.

I ran and grabbed my electrician's pliers, and making sure my hands were on the insulated grips, cut the parallel cables. By the time everything cooled down, the batteries had blown holes in the battery tops, and all the insulation on the parallel cables had been melted or burned off.

I still don't know exactly what the cause was. I did eventually discover that my WFCO converter would never go into trickle mode (factory specs say trickle after 44 hours), and would stay at 13.7V forever. I suspect a cell in one battery went bad, but have no proof.

I replaced the batteries with 2 Interstate GC-2 232AH 6V batteries ($150 for the pair with the core return) from Costco. I never have to worry about uneven battery charges causing fires again. I also replaced the converter/distribution panel with a PD 4135 after reading all the issues with WFCO converters. The GC-2s have been installed for 3 years, give me more capacity, and cost less. The PD converter goes into all 3 modes readily, and holds bulk charge much longer than the WFCO did. The PD fan is also a lot quieter. I also added a marine battery cut-off switch that I had bought but had not installed before. And I added a digital voltmeter ($7) with on/off switch beside the stereo just inside the door to make monitoring easier.

I am again a happy camper. But I do check battery water levels before each trip. And check battery/converter voltages once a day when camping.

Fred W
2014 Rockwood A-frame
2008 Hyundai Entourage
camping Colorado and adjacent states one weekend at a time

[gordonsick]

The technical reports that I cited have found it to be safe to connect batteries of the same nominal voltage in parallel, even if they have a different charge level. One cycle through the charging tends to balance them out. Presumably, you had your batteries in parallel, since they were both 12v.

The same won't hold for batteries in series, which is how you must connect your two replacement 6v golf cart batteries. In this case, you should check the voltage of each them, to make sure they are almost the same. Otherwise, you need to find a way to drain the high voltage one through a lightbulb or resistor until they have the same voltage.

–Gordon

[pgandw]

Originally Posted by gordonsick

The technical reports that I cited have found it to be safe to connect batteries of the same nominal voltage in parallel, even if they have a different charge level. One cycle through the charging tends to balance them out. Presumably, you had your batteries in parallel, since they were both 12v.

The same won't hold for batteries in series, which is how you must connect your two replacement 6v golf cart batteries. In this case, you should check the voltage of each them, to make sure they are almost the same. Otherwise, you need to find a way to drain the high voltage one through a lightbulb or resistor until they have the same voltage.

–Gordon

Gordon

I believe you have a misunderstanding of parallel and series circuits.

With batteries in parallel (2 12V batteries), the voltage across the 2 batteries is forced to be the same. If the 2 batteries in parallel are not at the same voltage, the second they are connected current flows from the higher voltage into the lower voltage battery in an attempt to equalize the two. The only limit on the current (there usually isn't a fuse in the parallel wires) from the higher voltage battery is the resistance of the wire linking the two, and the ability of the battery to produce or receive current. In my case, that current could easily have been 50A (2,500 watts of heat) on the initial surge with a battery difference of 1V.

Even when parallel batteries are nominally matched, any voltage difference between the 2 results in a current from the battery with more charge into the battery with less charge. The closer the batteries are matched, the less this equalizing current (which does nothing to power your 12V devices) will be.

The implication is that if a cell in one of the 12V batteries shorts out, or the battery won't hold a charge, the remaining "good" battery will rapidly deplete itself trying to equalize with the defective battery.

The way to prevent the equalization from causing the bad battery to drain the good battery is to not hook them in parallel. Use the batteries sequentially instead. Using the 12V batteries sequentially has the drawback of not providing as much capacity because batteries are non-linear in their discharges. You will have less AH if your load is 12A compared to 6A (12A split between the 2 batteries).

When 2 6V batteries are placed in series, the current through the 2 batteries is forced to be the same. The voltages of the 2 batteries can be wildly different, but the current through them and to the rest of the 12V system will be the same. With series 6V batteries, when one battery fails the whole system voltage drops, usually below usable. The bad battery does not take the good one with it, but most 12V devices won't work at 10V or less.

After the fire with parallel batteries, I'll take the risk of having my 12V system shut down because of a 6V battery failure. With my A-frame popping up and down manually, and a continuous run fridge with no DC control board, loss of 12V is not critical except to activate the trailer brakes during an accidental disconnect while towing.

hope this helps
Fred W
2014 Rockwood A122 A-frame with 2 GC-2 6V batteries
2008 Hyundai Entourage minivan
camping Colorado and adjacent states one weekend at a time

[gordonsick]

Originally Posted by pgandw

Gordon

I believe you have a misunderstanding of parallel and series circuits.

With batteries in parallel (2 12V batteries), the voltage across the 2 batteries is forced to be the same. If the 2 batteries in parallel are not at the same voltage, the second they are connected current flows from the higher voltage into the lower voltage battery in an attempt to equalize the two. The only limit on the current (there usually isn't a fuse in the parallel wires) from the higher voltage battery is the resistance of the wire linking the two, and the ability of the battery to produce or receive current. In my case, that current could easily have been 50A (2,500 watts of heat) on the initial surge with a battery difference of 1V.

Even when parallel batteries are nominally matched, any voltage difference between the 2 results in a current from the battery with more charge into the battery with less charge. The closer the batteries are matched, the less this equalizing current (which does nothing to power your 12V devices) will be.
[snip]

hope this helps
Fred W
2014 Rockwood A122 A-frame with 2 GC-2 6V batteries
2008 Hyundai Entourage minivan
camping Colorado and adjacent states one weekend at a time

Fred,
I don't think you read my earlier post, nor the papers I cited. Note that the papers were the result of careful research by engineers at firms that sell or build batteries. They were testing a large variety of unusual situations and found the batteries to be a lot more robust than posters in this forum claim.

If you have two batteries of the same nominal voltage, but one is not fully charged because you are bringing an additional battery to the circuit, then one cycle of the discharge and recharge will bring them back to equal status. That scenario is explicitly tested in the papers I cited. As I recall the poster I was helping had gone to a dealer to add an extra 12v battery, so that is why I offered my comment.

Obviously if you have damaged batteries, such as dead cells, you can't recover the dead cell in this manner. So, if then process of bringing a new battery in parallel to the circuit and running through the cycle doesn't result in equal voltages, you have some evidence of a bad battery that needs placing.

–Gordon