# What is the best way to combine multiple batteries (flooded lead acid)?

I wish to know the best way to connect four 12V 150Ah lead acid batteries to make it 24V 150Ah battery bank. I currently have connected 1 and 2 and 3 and 4 serially to make two 24V 150Ah volt banks, and then both banks connected in parallel to make a 24V 300Ah system.

But I feel that a better system would be to first connect 1 and 2 and 3 and 4 in parallel to make 12V 300Ah banks, and then connect them in serial to make 24 300Ah.

The battery bank is charged by 24V solar panels, charged during the day and used at night.

Is there any difference between the two connection systems?

• Either way you need to ensure balanced charging and discharging of the batteries. What are you considering? Commented May 1, 2019 at 4:08
• The batteries are charged using a MPPT charge controller 60A, 24V) and is connected to an 1500VA, 24 V inverter. After use over night , in the next morning I noted that the voltage of the batteries where the terminals of the inverter are connected have voltage 1 to 1.5 V lower than the other. At the same time the voltage across the 24V banks same. I think, since they are connected serially, there is a chance for a reverse flow of current within each 24V bank which may damage the battery. I assume that if the connection within the sub bank is parellel, this can be avoided. Commented May 1, 2019 at 4:16
• Parallel series or series parallel - there will be differences between the batteries. Is that mppt charge controller for the dolar panels? Show adiagram and update your question. Commented May 1, 2019 at 4:21
• Can you draw a diagram showing the two alternate connection methods you propose? There is really only one way to connect 4 12 volt batteries to get a 24V battery. I think both of your proposed methods are actually the same. Commented May 1, 2019 at 4:48
• I'd suggest you connect all four in parallel to begin with. Then wait 24 hours for the terminal voltages to stabilize and bring the charge level to about the same in all of them. Then split the pack into 2 groups of two in parallel and connect these in series. You should then be good to go. Commented May 1, 2019 at 5:07

To make a 24v system from 12v batteries, you need 2 in series. You have 4 batteries, so you will have 2 paths in parallel. So the only remaining option is whether you have this optional link, indicated by a switch, in place.

simulate this circuit – Schematic created using CircuitLab

Switch closed, you have two sets of parallel batteries in series. Switch open, you have two strings of series batteries in parallel.

In normal operation, with balanced batteries, and equal lengths of cable connecting them, the switch will have zero volts across it if open, and carry zero current if closed. In normal operation then, it really doesn't matter.

When that link does become significant is for balance fault conditions. What do you want your system to do should one battery fail open circuit, or short circuit, or wear out, or have one cell fail short circuit?

My preference is to have the switch open. It means one fuse per series string will protect all the batteries. It simplifies the wiring, having fewer connections, and guaranteeing that any wire sized for one battery is not going to accidentally carry two battery's worth of current. It simplifies debugging, as a DVM placed across the switch position will show a big voltage if there is an unbalance problem.

However, if you are (unwisely) using a bunch of batteries of mixed age, then the balance currents flowing through the switch may make for a better overall battery performance. If you're going to do this, place one fuse per battery, to protect against all the possible fault conditions.

• Thanks for the answer. Does it mean that first series then parallel ( as i did) is the best way ?. I also tested the voltage across the terminals where you suggested to place a switch. the DVM reads zero voltage in both way. Current flow not checked. The series connection wires are of same length. but one of the parallel wire is longer than other. Also all the four batteries are of the same make, age and capacity. Commented May 1, 2019 at 13:28
• @RajuVK Yes, the way you described in your first paragraph, which is the same as my 'switch open', seems to me to have more to recommend it than the other way, I always hesitate to use the word 'best'. Commented May 1, 2019 at 16:31
• What do you mean by the switch SW1 in the diagram ? is it simply a normal push to on type switch or is it any kind of circuit ?. Can I use a jumper cable with alligator clips connecting positive of bat 3 to negative of bat 2 instead of the switch ? Does n't it make bat1 and bat2 parallelaly connected making 12 V battery ( Sorry if the question seem to be silly . I not an engineer) Commented May 3, 2019 at 5:36
• @RajuVK The switch is conceptual, to indicate that you can have a connection there, or not. With no connection there, then it's equivalent to putting 2 sets of batteries in series, then putting those sets in parallel, your first paragraph. With a connection, then it's equivalent to putting 2 sets of two batteries in parallel, then putting those sets in series, your second paragraph. Using a switch is a way to draw both your configurations, showing the fact that although they seem different, they are fundamentally the same. Do not include this switch when you connect your batteries. Commented May 3, 2019 at 5:46
• I started a new thread on the subject with a problem I am facing electronics.stackexchange.com/questions/437689/… Commented May 9, 2019 at 10:49