Parallel versus Series battery bank

HarryS

New to off-grid. Need help on best wiring configuration. I have a 24V setup with 2 Xantrex 4024 SW+ in series. I am increasing the battery bank from 8 US Battery AGM 390 Ah (6V) to 16 batteries. I read an artcle from someone else who had wired 12 strings all in serieswith each bank hooked tobus bars. He talked about the problems with parallel/series setups. He had increasef his battery bank and had only series strings. Is this a good way to configure?

Cariboocoot

Re: Parallel versus Series battery bank

Welcome to the forum.

You're sort of mixing up terms and functions here.

You have a 24 Volt system. You have 6 Volt batteries. Therefor you must have four batteries in series in order to get 24 Volts.

Paralleling additional strings four batteries increases the Amp hour capacity. If the batteries are 220 Amp hours then two strings in parallel gives you 440 Amp hours at 24 Volts.

The issue is that multiple parallel strings give rise to potential problems with current sharing. The option is to instead use batteries of higher Amp hour capacity to begin with so that many parallel connections are not needed.

Short piece on battery banks: http://forum.solar-electric.com/showthread.php?15989-Battery-System-Voltages-and-equivalent-power

HarryS

Re: Parallel versus Series battery bank

Understand what you said. Guess I wasn't very clear. I want to expand my bank to sixteen batteries. Each is 390 amps and they are AGM. My question is would 4 strings of 4 batteries each connected in strings and each string connected to bus bars be a viable alternative to the standard series/parallel wiring. Also would i need a second charge controller. I have a Xantrex MPPT controller now. Thanks for the posts to read.

vtmaps

Re: Parallel versus Series battery bank

HarryS wrote: »

Understand what you said. Guess I wasn't very clear. I want to expand my bank to sixteen batteries. Each is 390 amps and they are AGM. My question is would 4 strings of 4 batteries each connected in strings and each string connected to bus bars be a viable alternative to the standard series/parallel wiring. Also would i need a second charge controller. I have a Xantrex MPPT controller now. Thanks for the posts to read.

Bad idea. Trying to charge batteries in parallel is fraught with peril, especially with AGM batteries. AGMs have lower internal resistance and are more sensitive to resistance in the cables and connectors. Also AGMs are more difficult to monitor because you can't measure their SG.

There is a short discussion of parallel batteries here:
http://www.wind-sun.com/ForumVB/showthread.php?14674

There is a link in there to the smartgauge site which has diagrams of how to hook up four strings if you must.

--vtMaps

Cariboocoot

Re: Parallel versus Series battery bank

HarryS wrote: »

Understand what you said. Guess I wasn't very clear. I want to expand my bank to sixteen batteries. Each is 390 amps and they are AGM. My question is would 4 strings of 4 batteries each connected in strings and each string connected to bus bars be a viable alternative to the standard series/parallel wiring. Also would i need a second charge controller. I have a Xantrex MPPT controller now. Thanks for the posts to read.

Four strings of four batteries each is series/parallel wiring.

You do not want to increase your battery bank to sixteen batteries, you want to increase you battery bank to 'X' Amp hours. In addition to the problems that can crop up with multiple parallel strings you have the additional issue of adding new batteries to old, which is also not a good idea.

If you did have four parallel strings of 390 Amp hours each you would have 1560 Amp hours. No single charge controller can flow enough current to charge that much. At 24 Volts you would need 4862 Watts of PV on two 80 Amp controllers.

Perhaps we should examine why you want to do this. At 50% DOD that battery bank would be approximately 18 kW hours of power. Rather a lot for an off-grid system, and rather a lot for 24 Volts. If your power demands are that large you should be thinking about upping the system Voltage to 48; reduces the number of parallel connections and increases efficiency somewhat. The down side is having to replace the inverters. Are these old SW series wired in series for 240 Volts? If so they are near the end of their lifespan and there are single units which can supply 240 Volts at 4 kW (unless you mean wired in parallel for 8kW - again their is a single inverter that can provide that).

inetdog

Re: Parallel versus Series battery bank

HarryS wrote: »

New to off-grid. Need help on best wiring configuration. I have a 24V setup with 2 Xantrex 4024 SW+ in series. I am increasing the battery bank from 8 US Battery AGM 390 Ah (6V) to 16 batteries. I read an artcle from someone else who had wired 12 strings all in serieswith each bank hooked tobus bars. He talked about the problems with parallel/series setups. He had increasef his battery bank and had only series strings. Is this a good way to configure?

You also cannot put your Xantrex 4024s in series. They each need 24 volts on the DC input side so they have to be in parallel there. Maybe you mean that they are linked together to provide 120/240 single phase three wire on the AC side with each 2024 supplying only 120V and the two being linked by a synchronizing cable?