Charging BIG 6 volt batteries.

[Tigerstripe40]

I was gifted three 6 Volt deep cycle batteries.
They are 6V-370S lead acid batteries.
About 360 amp hours each (20 hours).

They have been sitting for a while and are mostly discharged, But I don't know if they are any good.
I tested them with my meter, 2 of them come in about 2.8 volts, the other 3.2 volts.

My battery charger is a standard automotive battery charger but it does not have a 6 volt setting. I figured I'd series the 2 batteries that are the closest in charge, and put them on the automotive charger overnight. How should I charge the third so that I can test them all?

 

[Fast Eddy]

A couple hours on each of the three different possible pairs in rotation and note the voltage on each battery at each test. If you charge any one all the way to start this won't work, you'll get one hot battery and one that won't seem to charge. Charge on the charger's lowest setting.

 

[george_tlc]

Charging mismatched 6V batteries with 12V is a bad idea. It's the same problems as trying to charge two 12V series batteries in a 24V diesel when one battery is mismatched.

Do yourself a favour and either get a 6V specific charger or visit some local battery wholesalers and see if they'd let you try and use one of their 6V chargers to verify if the batteries are worth trying to recover.

cheers,
george.

 

[1911]

Charging mismatched 6V batteries with 12V is a bad idea. It's the same problems as trying to charge two 12V series batteries in a 24V diesel when one battery is mismatched.

Do yourself a favour and either get a 6V specific charger or visit some local battery wholesalers and see if they'd let you try and use one of their 6V chargers to verify if the batteries are worth trying to recover.

+1, I have some motorcycles that run 6-volt systems. A 6-volt charger is a necessity.

 

[Tigerstripe40]

I got a 6 volt charger, and some electolyte (they were all low)
Filled them all up and then put the charger on them. 2 of the 3 were bad.
Since another 6 volt version of the same battery is almost $300, and I REALLY don't need 370 amp hours, I just took them all in for recycling and bought a new 110 amp hour deep cycle 12v battery.
Thanks for your input.

 

[TrickyT]

You shouldn't add electrolyte to used batteries, since it changes the acid concentration. The liquid level in your cells may have been low, but what was missing was just water and hence you should just use water to fill them again. Water is lost when you charge them from the H2O disassociating into H and O2, both of which are gases and escape through the vent holes.

 

[george_tlc]

^ agree. You add distilled water to a low cell - unless electrolyte actually leaked out (upside down battery spill etc).

cheers,
george.

 

[DX FJC]

Batteries will charge and work in series even if they are different, but your set would only be good as your weakest cell. In series electrically it becomes one battery. You can even mix voltages. The amperage of your weakest battery is what the set becomes. Say there is one 900A@6V and one 700A@6V in series that would be 700A@12V. In parallel the amperage is the sum of the set as the voltage stays the same. With the same batteries it would be 1600A@6V.
Charging any battery you want to have around a quarter volt push per cell when charging. You do have some wiggle room with charging voltages, AGM being the lowest, maintenance free in the middle, and regular old lead acid at the top end. I use 3 x 6v in series for my amplifiers and charge them at 21vdc.

 

[george_tlc]

Batteries will charge and work in series even if they are different, but your set would only be good as your weakest cell. In series electrically it becomes one battery. You can even mix voltages. The amperage of your weakest battery is what the set becomes. Say there is one 900A@6V and one 700A@6V in series that would be 700A@12V. In parallel the amperage is the sum of the set as the voltage stays the same. With the same batteries it would be 1600A@6V.
Charging any battery you want to have around a quarter volt push per cell when charging. You do have some wiggle room with charging voltages, AGM being the lowest, maintenance free in the middle, and regular old lead acid at the top end. I use 3 x 6v in series for my amplifiers and charge them at 21vdc.

I would suggest that series connecting mismatched batteries is a BAD idea. Plenty of experience in the diesel/24V subforum. If you have one battery that is more discharged than the other, you will have serious problems when charging, one battery will overcharge/boil the other will not take a charge. The only way to get back to a working setup is to charge the batteries SEPARATELY to full charge.

Connecting them in series does not make "one battery", you still have two batteries with different state of charge. The OP was 2 batteries that were sitting as some "mostly discharged" state, not necessarily the same state of charge. Suggesting you could connect two different capacity batteries charge/discharge them is asking for balancing issues over time.

I (and many others) have plenty of experience with a 24V diesel system and charging issues that occur if one battery is drained down more than the other. The drained battery will never take on a full charge if you charge them in series, it requires a battery charger to take EACH battery back to full charge. The reason a multi-cell battery (such as a 12V SLA with 6 cells) works as "one battery" is that the cells are matched, same capacity, age the same way, sustain the same charge/discharge cycles (depth of discharge) and are exposed to the same temperature. Even then near end of life it is typical to see a "weak cell" that takes out the otherwise still good multi-cell battery.

I have no idea what you mean by "1/4 volt push per cell...". SLA/AGM batteries are charged at constant current (current limit)/constant voltage (if done correctly), they go through various stages of charging (prequal, acceptance, bulk etc) with float as the final state. There is no 1/4V push going on...

cheers,
george.

 

[DX FJC]

Series connecting different voltage batteries, not state if charge. Any combination of 2, 6, 8, 12, 16 volt batteries.

Remember that you only have one B+ and one B- so it is electrically one battery or X number of cells in series. In series one bad cell will take out the set even if you have 100 batteries. If one bad cell takes out a parallel bank then it needs to be speced with more capacity. My fire equipment has Series 60 Detroit Diesels, all 12 volt and starters draw 450a and charge at 360a. Yet they have 6 12v BCI #31 all in parallel. That's in the neighborhood of 6,000 amps. I can take half of the bank away and it will still function properly.

Each cell has a full charge of 2.1 vdc per cell. A 6v has 3 cells for 6.3vdc, a 12v has 6 cells for 12.6vdc, 24v = 12×2.1v=25.2vdc etc...

Charging... Regardless of what the voltage system will equate to about ¼v per cell charging voltage multiplied by whatever batteries (cells) it takes to get your system voltage to the desired volts. (12v, 6 cells × 0.25v= 1.5v + 12.6 = 14.1v). This is for system voltage charge rates, it does not change for parallel banks, only for how many cells it takes to get the system voltage. You can apply this to any charging system on or off of a vehicle or piece of equipment.

 

[george_tlc]

The OP had 2 6V discharged cells, at some 'state'... It was recommended to charge them separately with a 6V charger, do you disagree with that recommendation??


For SLA/AGM, cell charging is not 1/4V per cell etc, what you have is a poor rule of thumb. Correct cell charging is constant current transitioning to constant voltage through various states and then to float. To do it properly it also should be temperature compensated. Read some SLA/AGM datasheets.

A vehicle's charging system is a compromise since it misses many of the steps but mostly gets away with it since charging only occurs for a number of hours versus 24/7/365.

There is a reason balance chargers exist for various batteries (charging individual series connected batteries). Charging mismatched capacity batteries in series is a bad idea and will lead to early failures. Maintenance charging long term should be done 'per' battery versus all of them in series in that case.

A typical starting only situation is NOTHING like deeper discharging due to 4wding camping equipment use. You mentioned battery 'amperage', with 700A and 900A so that is a poor example since what counts is their actual AMP.HOUR rating and I'll state again that it is not a good idea to connect different capacity batteries in series and then discharge/charge them as if they were a single higher voltage battery.

I do understand batteries and how they need to be charged, one of benefits of being an ee. I am just correcting the statement you made that you can mix/match series batteries of different 'amperage' (your own example was 700A and 900A) and by implication not have any issues long term after many discharge/charge cycles.

Your detroit diesel example is a bunch on parallel batteries. Parallel batteries all see the same charge voltage/float voltage etc, so they will not have charging voltage discrepancies per battery. Series batteries (since they are not identical twins, especially in your 700A/900A example) will over time reach different voltages, i.e. if you had 2 series 12V batteries charging at 28.8V it is feasible that you will NOT measure 14.4V across each battery. Many owners of 24V diesels will connect a 12V good quality maintenance charger across each battery every few weeks to ensure they are topped up equally to 'reset' the state of charge mismatch that can creep in and to try an ensure maximum life out of their series connected batteries.

cheers,
george.

 

[DX FJC]

Everyone agrees that a 6v charger would be great, even in the original question otherwise he would not of said I do not have one.
I am just giving some knowledge on custom applications. Take it or leave it. I have literally taught classes on the subject, have over 20 years in the diesel field, and apply it to million plus dollar equipment.... Guess I was wrong.

 

[george_tlc]

Yes, unfortunately you are wrong. What you say is fine with parallel batteries, it is incorrect for series connected batteries that are mismatched capacity, age, state of charge etc.

cheers,
george.