Battery Pack Testing/Status

Hello,

I have a Taylor Dunn electric utility cart that has six 6V golf cart batteries (flooded lead acid).  I'm looking to understand the condition of each battery and the bank as a whole.  Can someone point me to information about how to test & understand 6V FLA batteries?

Thanks,
Jerry

The gold standard for checking flooded lead-acid batteries is looking at the specific gravity of the acid.  Use a battery hydrometer like this one.  You unscrew the caps of each cell, and suck up enough electrolyte to flood the internal bulb.  The bulb will float at the density of the acid.  Because the density of the acid can change with the ambient temperature, a temperature-correcting model like the one shown will be the most accurate.

A fully charged battery will have specific gravity (SG) of ~1.265-1.275.  Check with your manufacturer at what density they started out at.

You want to fully charge each battery first before making measurements.  Once charged, unscrew the electrolyte caps and measure the electrolyte SG.  Cell to cell and battery to battery differences should be less than 0.025 SG.  If greater than that, they need to be desulfinated, or equilized.  Your charger should have a setting for that.

If after being fully charged, and desulfinated, if the densities are less than 1.265, then the batteries have lost some capacity.  How much permanent loss depends on how badly the batteries have been treated in the past.  See what your manufacturer recommends as to when replacement is necessary.

Hi Jerry,
I don't know your level of familiarity with batteries or what gear you have, so I will offer a couple levels of answer.

First, has range decreased? Any other observations on performance of the cart?

Most car stores are equipped to test batteries, though typically 12V. Taking them to an AutoZone would be the lowest level checkout.

If you have a decent multimeter, which is a super versatile tool, check the voltages on each battery after use and after charge. It's pretty important that each battery have the same voltage, and that they track together across charge and discharge. Be sure you have the meter in DC mode, some (Klein Tools, specifically) combine VDC and VAC on the same dial setting and you use the 2nd key to access DC.

Lastly, there are actual battery testers that measure current draw under load, voltage sag when loaded, and other parameters, but you probably don't need these.

A decent lead acid charging chart is here: https://blog.ecoflow.com/us/battery-voltage-chart/

You typically want to stay above 50-70% charge, depending on whether the batteries are "deep cycle" lead acids.

Final note, don't swap for Lithium without rebuilding the charging and discharge system. Lithium chemistries overheat when asked to run at lead acid currents.

Best regards,
Mark

get yourself a load tester and disconnect each battery to test. carbon pile load testers are the best but ones with 100 amp element can be had for as low as $20USD

My own foray into using (and hopefully not too badly abusing) 6V FLA batteries in a 36V golf cart has been documented briefly in these two links, the idea being adding a solar panel for charging the batteries:

https://permies.com/t/80/24710/Solar-Electric-UTV-Polaris-sucks     [starting around Oct. 2023 entry]

https://permies.com/t/221392/PV-panel-hook

The used cart came with a 36V charger, but was sold to me with a mish-mash of 6 X 6V batteries ranging from 225 Ah to 185 Ah.  No-load testing was done on each by using a 6/12V auto-detect automatic charger (Schumacher brand) to bring each up to their maximum charge, then allowed to sit to see if fast-dischargers existed that should be replaced.  I was able to find two low-use 210 Ah replacements for two 'bad' batteries in the mix.  Nevertheless, I could not really tell you at what point the total bank is of no use because it is constantly recharging in the sun by day to 100% on the meter and for my use never drops below 50% as a resting state charge. (Stored in the winter months, the bank is charged with the provided 36V charger every few weeks to guard against battery freezing and further decline.) I've tried to be diligent with battery maintenance (water level, cable and terminal cleaning, etc.) in hopes of squeezing everything I can out of these batteries until they may need replacing.....and then may finally switch to LiFePO4 batteries if the cost is right. For what it's worth, the build that I ended up with continues to impress......for my needs with the cart, no need to use the plug in charger at all in the summer 'season' (loosely, summer and the tail ends of spring and fall).  All battery charging done by the sun with the single roof panel run through an MPPT charge controller.

This doesn't really answer your question head-on, but I would say that doing what you can towards keeping the batteries charged and up on water might be positive steps in extending the life and performance of your system.  Keeping them as balanced with each other with respect to charge/discharge levels seems to be a desired goal as well. Good luck!

To take this one step further I'm considering adding 2 more 6V batteries as my motor controller will support 36V & 48V.  I have 6 GC2 batteries from another 36V application that are maybe 10 years old, but had very light use.  How would I pick the best 2 to add to the existing 6 batteries to give me a 48V system?  I know it isn't ideal to mix battery ages in a bank, but would there be any significant problems in doing this?

I did order the hydrometer and I have a multi-meter.

Hm. 10yrs old is a caution. I'd pick the best by charging and seeing which ones hold voltage best. After a charge, the voltage will decay back to some resting level that reflects the health of the battery, and it will supply some amount of charge (amp hours), and those two data points give a pretty good picture of the battery. The time it takes to charge or discharge is a good proxy for the capacity. The listed cranking amps will also serve as a more rough proxy, but ageing of the battery will affect this.

I caution that series strings (like 6each strung to get 36V) does not guarantee that each battery has the same voltage, but they all have to pass the same current. They do this best if they are all equally charged. However, if they are dissimilar, you can easily get to 36V total with, say 5ea at 6.5V and one at 3.5V. This string will not perform very well, as the weak link won't contribute much current. Battery balancers alleviate this, though I am not familiar with 6V flavors, but have used 12V balancers with good results. But you want comparable batteries in a series string, or the weak link will limit the chain.

Happy homesteading!
Mark

Right now I have a 12V 3-bank charger (charges batteries in pairs).  I have found a 4-bank 6V/12V charger, I'm considering getting two of these then each battery will be charged independently, but the load would draw from all of the batteries connected in series (either 6 or 8 batteries).

Jerry Ward wrote:Right now I have a 12V 3-bank charger (charges batteries in pairs).  I have found a 4-bank 6V/12V charger, I'm considering getting two of these then each battery will be charged independently, but the load would draw from all of the batteries connected in series (either 6 or 8 batteries).

What you are trying to say here is quite unclear to me.  Could you please go into more detail as to what you are attempting?  When you say "12V 3-bank charger (charges batteries in pairs)" are you saying that there are 3 parallel strings of two 6V batteries wired in series?  A shorthand way to write that would be 2S3P.

Batteries want to be charged at a certain amperage rate, and the amperage applied would be divided between each parallel string.  To give an example, let's say the batteries in your strings are Trojan 6V golf-cart batteries.  The Trojan 6V is rated at 225Ah.  Trojan's website states their battery likes charging at a 13% rate, so that's 225Ah X 0.13 = 29.25A.  Let's just say 30A to make it easy to remember.  Larger or smaller brands will of course have their own best charging amps.

Now, if you have two parallel strings of those batteries (two in series for 12V), that means you need 60A if you are charging both at the same time.  Three in parallel would be 90A.

Getting back to your specific situation, it looks like what you need to do is break up your 36V battery bank and make three 12V strings for charging.  Once fully charged, start measuring the electrolyte with the the hydrometer I mentioned above, and base decisions about battery health on what the specific gravity is.  This is the best, most accurate way to determine battery health.  Do you know what the amperage rating of your charger is?  Base the number of strings based on how many amps the charger can pump out.

The charger I have has 3 independent circuits that charge 3 different 12V batteries.  In my case, I have six 6V batteries in series.  Each one of the circuits of the charger is connected to two 6V batteries.

This helps prevent an imbalance of six batteries in series, not completely as it charges the batteries in pairs.  Thus my considering new chargers with a circuit for each battery.

My concern is one of these older batteries will limit the current flow of the whole series.