3x12v SLA battery pack BMS

[joshcarr]

I'm making a powerwheel for my little cousin and I've got three 12v sla batteries I want to use in parallel. Do I need a BMS to charge them? Can I just charge them while still in parallel or would charging them all separately be best? Thanks.

[metrologist]

What are these batteries? The general recommendation for solar SLA is to use two 6V golf cart batteries in series, because they are typically designed to be very deep discharge cells.

Most automotive installation just use a battery isolator.

[paulca]

Not sure what a power wheel is, but 3x12V SLAs in parallel shouldn't be a problem.  There are caveats though.  They should be similar capacity, similar age, and importantly, when first connected, similar resting voltage.

You can charge 3 in parallel with a normal SLA charger (or car battery charger but check the max current of the batteries).

That said, I just added a 12Ah to my 26Ah SLA in parallel for my solar set up and I don't expect any issues.

[joshcarr]

Power wheel  is like those barbie jeeps and little mustangs that you can get for kids. Basically very slow electric car lol The batteries were initially in a e-bike battery until I switched to Li-ion. That is what i was suspecting paulca, and they are all identical since they were in a pack together. Thanks guys.

[Gregg]

Lead acid batteries are very forgiving and as long as there are no bad cells in the set they can be charged in parallel with a voltage limited charger.  All lead acid batteries like to be kept in a float stage of approximately 13.4 to 13.6 volts for a nominal 12 volt battery.  The float voltage is dependent on temperature and exact battery chemistry; try to find manufacturer’s specs.  The problem with any series / parallel setup of cells is the possibility of reverse charging of a weak cell when the system is in a discharge state. None of the types of lead acid batteries like to be discharged too deeply and paralleling them exacerbates the problem of possibly reverse charging one of the cells.  There are cheap battery undervoltage modules like ebay item 132406444865 that could be used with a suitable relay to disconnect the battery bank before they are damaged.

A discharge test is the best way to determine that all of your batteries are in pretty good shape. Charge them all up and individually start a discharge via something like an automotive headlight bulb and take voltage readings every minute for 5 or 6 minutes and compare the results.  If the readings are close you won’t have any problems using or charging them in parallel.  If one battery is sluggish, try giving it an equalize (raised voltage) charge and try it again to see if there is a noticeable improvement.  If an equalize charge makes it worse, it probably is low on electrolyte and probably isn’t worth using, although there are lots of youtubers that show how to open them and add electrolyte. 

My brother has more vehicles and toys that use 12 volt lead acid batteries than he can count keeps all of his inactive batteries on a float charge using a single power supply.  Many of his batteries are over 10 years old and still work. 
 
The telco industry uses tons (literally tons) of lead acid cells in series / parallel configurations.  Many large UPS systems parallel cabinets full of large 12V VRLA in series at about 500V.   

[joshcarr]

That's alot of good info. I'm running with an ESC which has a built in low voltage cut-off so it should be all good.