24v 120ah Battery Pack Current Flow

Thread Starter

jc0r

Joined Oct 15, 2013
14
Hi all

I'm currently making 2 x 24v 60ah battery packs from SLA cells I had spare. Each pack has 4 x 12v 30ah cells wired in series and parallel. The reason I am making 2 packs and not one is due to weight as these packs need to be transported.

I would like the packs to be duel purpose, as in, sometimes they will be used separately from one another, but other times I would like to make a link cable to join the 2 packs together producing 24v 120ah.

My question is in relation to DC flow. I've always understood DC current to be like the analogy of a water pipe, flowing in one direction, + -. That I understand. But how is flow affected if I was to have a load connected to both battery packs even though they were linked? Will this cause an imbalance on the cells? For example, let's say I have an 8 amp load connected to pack 1 and a 2 amp load connected to pack 2 but the 2 packs are linked. Can the electrons in the negative side flow in either direction to discharge the cells as a whole? Or, is it the case that I have to have all loads connected to one pack rather than both sides of the packs?

Thanks in advance
 

pmd34

Joined Feb 22, 2014
499
Hi jc0r, if they are only linked by a common negative terminal then the batteries will act independently. Only when the positives and negatives are connected will they be dependent on each other.

The problem with your idea is that if you use the battery packs independently, and then connect them together some time later to get your 24V 120AH, without some sort of diode the battery that has been discharged the most will get a very sudden rush of power and charge from the other.
 

Thread Starter

jc0r

Joined Oct 15, 2013
14
Hi jc0r, if they are only linked by a common negative terminal then the batteries will act independently. Only when the positives and negatives are connected will they be dependent on each other.

The problem with your idea is that if you use the battery packs independently, and then connect them together some time later to get your 24V 120AH, without some sort of diode the battery that has been discharged the most will get a very sudden rush of power and charge from the other.
Thanks for your reply. To make the 2 packs combine to 120ah, obviously I would need to connect both positive and negative terminals. This would make them dependant on each other. I'm not sure I'm explaining myself correctly. Can I connect a load to both packs though? So the electrons can travel both directions on the negative side, as well as the positive side to discharge the batteries equally?

Due to the reason you stated 2nd though, this may be a non starter
 

pmd34

Joined Feb 22, 2014
499
You effectively have to load the battery with the highest voltage first, until it reaches the same voltage as the other battery for direct connections. Although you could for example have 2 separate voltage regulators one from each battery, and then combine the outputs.
 

mvas

Joined Jun 19, 2017
538
Hi all

I'm currently making 2 x 24v 60ah battery packs from SLA cells I had spare. Each pack has 4 x 12v 30ah cells wired in series and parallel. The reason I am making 2 packs and not one is due to weight as these packs need to be transported.

I would like the packs to be duel purpose, as in, sometimes they will be used separately from one another, but other times I would like to make a link cable to join the 2 packs together producing 24v 120ah.

My question is in relation to DC flow. I've always understood DC current to be like the analogy of a water pipe, flowing in one direction, + -. That I understand. But how is flow affected if I was to have a load connected to both battery packs even though they were linked? Will this cause an imbalance on the cells? For example, let's say I have an 8 amp load connected to pack 1 and a 2 amp load connected to pack 2 but the 2 packs are linked. Can the electrons in the negative side flow in either direction to discharge the cells as a whole? Or, is it the case that I have to have all loads connected to one pack rather than both sides of the packs?

Thanks in advance
HOW can you have a load of 8 Amps on Bank #1 and a load of 2 Amps on Bank #2 - if both banks are in parallel = one big 24V Bank ?
Actually, you have 10 Amps on the THE 24V Bank.

You need to be extremely careful with placement of the inter-connect cables.
http://www.smartgauge.co.uk/batt_con.html

With all 8 batteries connected as one big 24V bank,
it will be very difficult ( nearly impossible ) to fully charge all 8 batteries to 100%,
without over-charging some and under-charging others = very short battery life.
 
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