12 volt battery tester

[Jenga Jack]

Looking for a recommendation on a basic battery tester for my 12 volt deep cycle battery. One that shows actual charge numbers, not just "good, average, low, dead".
All I want to know is the exact charge level of the battery when dry camping. To know when to hook up portable solar panel charger, then read again after a few hours of charging.

Thanks!

[nayther]

When you say "tester" are you looking for a Hygrometer, something that reads the specific gravity of the acid?



or do you mean "meter" if the former you have to remove a cap and test the electrolyte. The latter is a volt meter. A fully charged battery will be about 12.5-12.8 volts on a meter.

[Jenga Jack]

Originally Posted by nayther

or do you mean "meter" if the former you have to remove a cap and test the electrolyte. The latter is a volt meter. A fully charged battery will be about 12.5-12.8 volts on a meter.

I should have said "volt meter". Sorry, I'm not a car guy or an electrical guy.

[bikendan]

A multimeter is a must for all RVers and will work on both the 12v DC system and the 110v AC system.

Sent from my SM-G781U using Forest River Forums mobile app

[clintbonnie73]

If your battery is acid filled with removal caps, the only good way to check the condition is with a "Hydrometer" (check only after fully charging). If one cell or more cells check bad, replace the battery.. Get one a Napa Online or at their store. See:
https://www.napaonline.com/en/p/BK_7...pressionRank=2

[acadianbob]

You will not get a good voltage reading if the battery is under load.

[camaraderie]

JengaJack...the problem with readings from a VOLTMETER is that IN USE batteries and those recently charged will NOT provide a correct reading. The charge % charts you will find here and elsewhere all require a static battery.
What you REALLY want if you want to know what your % charge is, what you're drawing and WHEN you can stop charging or NEED to charge (50%)....is a battery monitor which will run you around $150 and is EASY to install. Search VICTRON on this formum and you'll see some examples. Don't buy any battery monitor of a different brand UNLESS you see something about Peukarts constant in the product description or manual. The Victron WILL pay for itself by saving your batteries AND saving you time and generator run time & fuel.
FWIW ...a 100 watt solar panel in full sun will deliver approx. 25 amp hours to your batteries in a DAY. A single group 27 battery holds about 100 amp hours and when half depleted will need you to put back 50amp hours. Most people use at least 2 of these when boondocking so you have about 100 amp hours to put back with your solar panels. That is 4 days of sun...OR more panels OR a generator needed.

[Larry-NC]

Originally Posted by Jenga Jack

I should have said "volt meter". Sorry, I'm not a car guy or an electrical guy.

Temperature effect
You can get a multimeter for less than $15, often less than $10. HFT used to give them away (still have four or five). They will measure battery voltage but it's not a perfect indicator of state of charge (SOC). For one thing, SOC is a function of temperature. You need to have a table that lists both temperature and voltage to ACCURATELY determine SOC. (I laminated one and kept it in the golf cart for a while.)

Surface Charge effect
Battery charging works like this: You can cause a chemical reaction where
Hydrogen and Water (H20) and Lead Sulfate (H2 plus PbO2 convert to pure lead (Pb) and sulfuric acid (H2SO4) if you pump energy into it. When you discharge the battery (recover the energy you stored in it), the reverse chemical reaction takes place.

But as a side effect of charging, extra electrons build up on the surface of the battery's internal plates. These give a false reading--too high. To get a reliable reading you can either wait a few hours after charging completes or simply apply a load to drain off the electrons. Just turn on the lights for a few minutes before taking your measurement.

Accuracy
For the best accuracy, don't use a multimeter. Instead measure the fluid to see whether it's water or Sulfuric Acid, or somewhere in between. One of these weighs more than the other (weight per fluid ounce, called "specific gravity"). A "hygrometer" will give you this information. The specific gravity measurement also varies with temperature, but hygrometers compensate for this.

It will look like a turkey baster. Take one of the battery caps off. Use it to suck some fluid. Take the reading. Squirt the fluid back in.

This is the first one I found on Amazon. There are probably better ones and worse ones. The research is left to you.

[Slow Moon]

This is designed for starting batteries, but it works for me on deep cycle batteries. It will give you a digital reading of cold cranking amps plus voltage and internal resistance. It makes carbon pile testers obsolete, in my experience. It's instructions say only one battery at a time, so I have to disconnect in multiple-battery applications, but it is amazingly quick, easy, and precise for determining battery condition.

https://www.harborfreight.com/digita...zer-66892.html

I have been using mine for many years and have not seen anything better. It is not a substitute for a battery monitor, which is what you need for power management during RV use.

Any comments on using this with deep cycle as opposed to starting batteries are welcome.

[mnoland30]

You do need a voltmeter and can get by with just that, but you're much better off with a battery monitor with a shunt that actually measures watts going in and out. That works under load, and is very accurate. Victron sells one at a reasonable price.

[NavyLCDR]

Originally Posted by Jenga Jack

Looking for a recommendation on a basic battery tester for my 12 volt deep cycle battery. One that shows actual charge numbers, not just "good, average, low, dead".
All I want to know is the exact charge level of the battery when dry camping. To know when to hook up portable solar panel charger, then read again after a few hours of charging.

Thanks!

This is what I have. I mounted it right in the top of my plastic battery box. You do need a jumper to go from negative battery terminal to the shunt. Once you have the jumper (and I think it is 2 1/8" hole saw), it takes less than 10 minutes to install. If you install it outside, you will want to run a bead of silicone caulking around it. It will show you capacity left in the battery (don't go below 50% with a lead acid battery!), the rate at which the battery is being charged or discharged (in amps), and the voltage present at the battery terminals.

https://www.amazon.com/AiLi-Voltmete.../dp/B07FGFFHC6