Thread Starter

jhumphreys

Joined Aug 22, 2017
8
I'm posting today hoping for some guidance on a project i would like to attempt. I recently came into a lot of 64 12v 7AH Sealed Lead Acid batteries from a backup UPS. The batteries are still good and take a charge and load like normal for the most part. Since i only have the batteries and not the UPS I am looking to build a charger so that i can maintain them in a string of maybe 8 at a time. I cannot charge them 1 at a time because i have run out of patience for this.
So what i am hoping for is a fairly simple variable power supply that goes up to around 110vdc.
Manufacturer specs on the cells are to recharge at 2.1volts per cell, float at 2.25 volts per cell and equalize at 3.1 volts per cell.
I think the easiest way is to buy a power supply but not the cheapest. Maybe?
I'm not looking for easy but cheapish and correct would be nice. And maybe this is a good chance to practice and refresh my electronics skills! Or perhaps my lack there of. Any Ideas, circuits, criticism would be greatly appreciated.
Or at least some ideas may keep me from the wrong path to start with.

Thanks, Joe
 

KeithWalker

Joined Jul 10, 2017
1,787
If you charge eight batteries in series, you are running a risk of over charging and ruining some of them. You need to monitor the terminal voltage of each battery and terminate the charge when the first one becomes fully charged. You could buy a data acquisition and control system to do this (quite expensive) or build your own. It would probably be much simpler and less expensive to use a 20V 20A DC supply and build eight 14.4V constant voltage units with 2.16A current limiters. They would not need monitoring and would automatically become trickle chargers when the batteries are fully charged.
 
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Thread Starter

jhumphreys

Joined Aug 22, 2017
8
If you charge eight batteries in series, you are running a risk of over charging and ruining some of them.
I am already able to charge individual batteries with single chargers. I am not interested in doing that.

I guess i should have said I am going to charge them 8 at a time. Just as the UPS did. As i have no way to monitor individual cell voltages only battery voltages (6 cells). I am going to have to take my chances. Since the batteries were free and headed to the junkyard, and since I have many I was thinking i would charge them and discharge them at the same rates. Then i could get data and try to match batteries to packs.

I hope my thinking is correct.
If i give the pack an equalize charge of 115.2vdc (14.4 x 8) or 2.4 volts per cell
and float them at 108v (13.5 x 8) 2.25 volts per cell
then discharge them as a pack at a certain rate then when battery voltages hit a certain voltage ( 1.75 or 1.8 volts per cell) it will give me a good idea of battery discharge rates and health?
Then i can match batteries and make packs?

battery pack.png
 

crutschow

Joined Mar 14, 2008
27,225
If the batteries are all the same type, rating, and brand, you could likely charge them all in parallel with a smart charger.
I would not recommend series charging.
 

KeithWalker

Joined Jul 10, 2017
1,787
If the batteries are all the same type, rating, and brand, you could likely charge them all in parallel with a smart charger.
I would not recommend series charging.
That would be a good way to go, but don't connect batteries with different terminal voltages together in parallel. That can cause a serious explosion!
 

Thread Starter

jhumphreys

Joined Aug 22, 2017
8
im trying to not confuse myself here but parallel charging? i have never done that. cells are charged in series. each 12v battery has 6 cells each at 2 to 2.5 volts depending on the battery charge. they are in series inside the battery and thats the way they charge.

Hopefully we can get back on track here. Im looking for ideas building the power supply for 120vdc. A variable dc power supplyj.
 

Ian0

Joined Aug 7, 2020
2,235
All the 108V DC Emergency lighting units out there charge nine 12V batteries in series, and they achieve 10 year battery life.
 

Thread Starter

jhumphreys

Joined Aug 22, 2017
8
It would probably be much simpler and less expensive to use a 20V 20A DC supply and build eight 14.4V constant voltage units with 2.16A current limiters. They would not need monitoring and would automatically become trickle chargers when the batteries are fully charged.
I think i like this idea also. If i could build one charger that can do multiple batteries. At the end of the day it solves my charging. However i have to take the bank apart to charge and then put back together for use.
 

MrChips

Joined Oct 2, 2009
23,535
Don't do it without monitoring each battery's voltage and temperature.
This is what happens to batteries charged in series and not monitored. In this case the "intelligent" charger failed to stop charging.

1618024085556.png
 

KeithWalker

Joined Jul 10, 2017
1,787

Ian0

Joined Aug 7, 2020
2,235
Don't do it without monitoring each battery's voltage and temperature.
This is what happens to batteries charged in series and not monitored. In this case the "intelligent" charger failed to stop charging.

View attachment 234941
It was probably so intelligent that it thought it was about time the customer bought some new batteries.
Three-state chargers tend to change from absorption (2450mV/cell) to float (2270mV/cell) when the current falls to 0.03C.
As batteries age the current in absorption charge doesn't fall as far and it reaches a point where it never gets as low as 0.03C. The charger stays in absorption mode at 2450mV/cell and ruins the batteries.
 

Thread Starter

jhumphreys

Joined Aug 22, 2017
8
Thanks to all who posted. I did some due diligence over the weekend and have decided not to attempt this. I have a couple chargers that charge single 12v batteries very well. At the moment i have no need for anything other than that. I came across a lot of information about power supply problems and proper design. At the end of the day i'm not able to make something good enough that is worth the effort vs the cost of a good power supply (maybe someday). I have found some great discussions on circuits to monitor battery charge. Some pretty good charge controllers, current limiting circuits, temperature monitoring and controllers to start and stop charging. I think I am going to spend some time on these and learn some more about the subject. If i am able to make one that i feel is safe, i suppose it could be scaled up later. Maybe I will have a controller to share at some point.
Thanks again
Joe
 
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