SLA batteries charging

[longsundayrun]

Hi again y'all !!....my MP 3 EV is running on 36 volts with three 12 volt, 18 ah SLA Powersonic batteries. When I first received the batteries, right out of the box, they tested at 39 volts !...I said, WOW that's cool....but after my first test run, I could not get them back to 39 volts when I charged them...only achieved about 36.5 .....does anybody have any idea what has happened ?....I did not deep discharge them...the MP3 controller won't let them go below 24 volts. I charge them with a 36 volt charger...do I need to charge them individually with a 12 volt charger or is this normal for SLAs to be so high when new and see a reduction after some use ?...Thanks !!!

[Bikemad]

Hi Ed,

Unfortunately, it would appear that your SLA batteries have already been discharged well below the recommended low voltage level.
A 6 cell 12V SLA should not be allowed to discharge below ~10.5V, therefore your 18 cell 36V battery's voltage should not really be allowed any lower than ~31.5V, which is a lot higher than 24V.

If your controller is allowing the battery to be used as low down as 24V (which I'm guessing was the voltage measured at the battery just after it cut out) I would say that it is probably still set to 24V (the standard setting) which will allow the battery voltage to fall as low as ~21V under load before the controller would actually cut out.

As your battery does not have its own battery management system (BMS) you really should have the controller set to 36V to protect the battery from being discharged too low.

Regarding the charging voltage, your 36V charger should have a maximum output voltage of somewhere between 41.5V and 43.5V, and I would normally expect a 36V SLA battery to reach at least 42V before the end of the charging cycle.

If your charger's maximum output voltage is below 41.5V, it will never be able to fully charge your battery.

If you have access to a 12V SLA Charger, you can try charging one battery at a time and see if the voltage increases to the expected maximum level of between 13.8V and 14.5V per battery.

It might be a good idea to check and record the individual voltages of each of the three batteries, so that you can see  if there is any obvious difference between the voltage readings from the three batteries before and after charging.

Alan
 

[longsundayrun]

Hi Alan !
Actually, my software parameters were set to 36 volts and it never actually cut out...we stopped the test around 30 volts....because we had enough test data to do our MPGe math. Anyway, My battery charger puts out 42.9 volts so I'm glad at least that is correct. But I will Check each battery individually tomorrow for differences as you suggest. But I thought SLA rechargeables could go down a bit under 30 without damage...thought I read down to about 70%, which would have been about 28 volts. Anyway thanks a bunch for the tidbits of info and I'll get back to you.
Cheers
Ed

[longsundayrun]

Hi Alan !....it seems I am able to achieve a full charge after all....I now have almost 38 volts after a charge....my old crappy VOM said the 36 as I mentioned, but a new one out of the box this morning here at school confirms the 12.6 volts per battery and the correct total...it seems the 39 at new was not accurate because of the defective VOM....whew!!...anyway, thanks, Alan for your help !