Number of batteries

[kj8olot]

Hi - new to the forum and looking for a little input. I have an engineering background with an ASEE - but spent my time in permanent and electromagnetic engineering - so - a bit rusty on my DC battery stuff...

Setting up an off grid system for back up, to start experimenting with alternative energy - I currently have a 48V DC panel array set up and will be adding a wind generator....The question I have is this - what's better - fewer batteries with higher voltage OR more batteries with lower voltage in an appropriate bank? I.e. - 8 - 6VDC in series OR 4 - 12VDC in series? Just an example - but what is the conventional thinking about these configurations AND what's better - AGM or wet cell? I am leaning towards AGM based on my reading...please advise - thanks!

[inetdog]

For high power you want 48V. That is the highest system voltage that can use unshielded terminals under the NEC. As a matter of design you will put insulating shields over the battery terminals, etc. anyway if they are easily accessible.

To get your 48 volts at the Amp Hour rating you need, you want to use a single series string of batteries if you can. That can mean using high capacity low voltage batteries, from 8V all the way down to 2V to make up the string. Do not get locked into 12V batteries and putting them in parallel.
The size of the individual batteries should be limited by what you can safely move for installation and maintenance. For example, do not get a 400# battery that needs a forklift unless you have a forklift and are qualified to use it.
If necessary to reach your needed power capacity you may need to go to two parallel strings, but that should not be your first choice. More than two strings in parallel -- forget it.

Wet cells (actually Flooded Lead Acid = FLA) will be less expensive and last longer than AGM. Only consider AGM is you are far north and have to be able to charge the batteries at high current with only a few hours of effective sunlight each day during the winter.

Read all of the sticky threads in the Off Grid sections for a much more complete description.

[kj8olot]

Thanks inetdog - I live in Michigan - 4.2 hours average - second to last to Washington state if I read the map correctly. Also will be supplemented with a wind generator...

so...with that - do you still recommend wet cell? I was of the impression that AGM lasts longer, and realize the maintenance issues with wet cell, although not a problem for me - would prefer not to deal with it.

I understand about the parallel strings - not to exceed 2 - as I understand the imbalance issue due to internal resistance, etc., right?

[Mike90250]

Generally, if you don't have to tie your hat on when you go outdoors, you don't have enough wind to bother with.

AGM batteries have shorter lifetimes than flooded batteries do. Flooded batteries need water 1x a month and can be equalized (unlike AGM) when the bank gets out of balance.

Figure your loads first, then that gives you the size of the battery bank required. A single series string of large 2 v batteries is preferred over parallel 12V batteries.

[jflorey2]

so...with that - do you still recommend wet cell? I was of the impression that AGM lasts longer . . . .

Not really. In some applications (UPS) they can last longer if there is no maintenance, but for someone like yourself flooded cells are the way to go. You are going to be abusing those batteries, and you can't add water later to an AGM battery.

I understand about the parallel strings - not to exceed 2 - as I understand the imbalance issue due to internal resistance, etc., right?

Sort of. The issue is that the mechanism whereby lead acid batteries balance themselves during cyclic use only works when they get a certain minimum amount of current. If everything is perfectly balanced then all strings will get the same current. However, if one cell gets a little weak, its resistance goes up and that entire string now gets less charge current, which means that one cell never gets fully charged, which makes its resistance even higher etc.

If you float the batteries forever (as in UPSes) then large parallel strings can work; eventually, just keeping batteries at float voltage will get them to full charge, although it will take a long time. But for solar applications one big string is ideal.

[adaviel]

To get your 48 volts at the Amp Hour rating you need, you want to use a single series string of batteries if you can. That can mean using high capacity low voltage batteries, from 8V all the way down to 2V to make up the string. Do not get locked into 12V batteries and putting them in parallel.

Can you elaborate on the reasons to avoid 12V and parallel connections ? Price ? Reliability ? It's quite common to run 12V in parallel for small marine applications, e.g. switched start/house/both.

[Sunking]

It's quite common to run 12V in parallel for small marine applications, e.g. switched start/house/both.

Very true and very ignorant. Like I said in your other thread batteries ar 2 volts. You are stuck inside a 12 volt box with the rest of the RV and Marine crowd. It has to do with everything you said. You cannot Balance the Battery Internal Resistance, cable and connector resistance. One of the strings in a parallel arrangement will have lower resistance forcing it to do most of the the work. That in turn shortens its cycle life and his paralleled dead beats who do not work. If you need 500 AH battery, buy 500 AH batteries. But you are not going to find 500 AH 12 volt batteries. They will be either 4 or 6 volts you wire in series. A 12 volt 500 AH battery would be a beast weighing in around 350 pounds. 6 volt 500 AH around 175 pounds, and 4 volt around 120 pounds.