48 v battery bank wiring help

new2PV · June 2015

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Hi, does anyone see anything wrong with this connection setup. 2 parallel strings of 12 volt 200ah batteries to make 48 volts. The view is an elevation view of my racking. I plan to have the parallel cables the same length but some jumpers will be 12" and some 30". I can attach a real life pic of the batteries for now in case its a bit confusing..

vtmaps · June 2015

Depending on how you mix and match the 12" and 30" interconnections, there may be a problem. I need to see a diagram with every interconnection labeled with length and gauge.

Also, the way you connect the two strings is wrong. The negative output (between battery 3 and 6) is midway between the two batteries. That does not match the positive output which is closer to battery 4 than to battery 5.

You should also make sure that both battery strings are at the same temperature... if they are on a rack with one string above the other, the top string may be warmer.

--vtMaps

Engr.abbas · June 2015

You should Take the Positive terminal from the upper and the negative from the lower battery.

inetdog · June 2015

First, carefully read the information at http://smartgauge.co.uk/batt_con.html for tips on wiring batteries or stacks of batteries in parallel.
Second, if you have not bought the batteries already, consider going with 8 6V batteries (at twice the AH) instead so that you have only one series string of 8 batteries.
Third, take vtmaps advice and draw it up as a schematic rather than a pictorial to make it easier to add up the wire lengths. You MUST have equal total wire length (all interconnects plus wiring to the common points) in the two strings. It does not matter whether a particular length of wire is at the = end, the - end or in the middle, just add them all up.
Why 36" between rows of batteries?

new2PV · June 2015

vtmaps wrote: »

Depending on how you mix and match the 12" and 30" interconnections, there may be a problem. I need to see a diagram with every interconnection labeled with length and gauge.

Also, the way you connect the two strings is wrong. The negative output (between battery 3 and 6) is midway between the two batteries. That does not match the positive output which is closer to battery 4 than to battery 5.

You should also make sure that both battery strings are at the same temperature... if they are on a rack with one string above the other, the top string may be warmer.

--vtMaps

In real life all the parallel cables will be the same length between strings I just made the made the sketch like that, sorry I should have mentioned it. The cables are going between shelves so some must be longer. The top four batteries are one string and the bottom 4 batteries are the 2nd string. If they are different temperatures what can I do to help that? See attached electrical schematic equivalent of the hook up.. image_7736.jpg I will look at the links you posted as well.

mike95490 · June 2015

new2PV wrote: »

.. The top four batteries are one string and the bottom 4 batteries are the 2nd string. If they are different temperatures what can I do to help that?......

Plan Ahead. Put the batteries on one level if possible, When Stacked over/under, you have thermal mismatch (upper row heated by bottom row) Adjusting the electrolyte SG is the only way to fix thermal issues. They produce "tropical" batteries with a slightly weaker acid, to counteract the more active warm electrolyte. I have no idea how to properly do that at home.

Also, checking fluid and SG will be harder, looking under the uppers, and leaning way over to check the uppers

softdown · June 2015

Though initially expensive, there are valid arguments for using 2/0 copper for battery interconnects. If you can manage it.

new2PV · June 2015

These are sealed batteries, but wet cell, I wont be measuring SG. I plan to use 4/0 interconnects, but I am not sure on what lengths I need now, since I don't know if I am doing it correctly according based on what the members here are posting. I will re-post the pictures of the battery shelves.
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softdown · June 2015

Those look a lot like 8Ds. If so, you can place four on every four foot shelf. They can hang over the edge a couple inches. Knock out the shelf that is second from the bottom. Then you have access to the batteries on the bottom shelf. Or....at least raise that shelf before placing batteries on it.

Vic · June 2015

Agree with mike, batteries really need to be on ONE level. This should minimize temperature differentials, and should tend to minimize total cable length. Inverters, especially large-ish ones can have stability issues when cable lengths are long.

One disadvantage that often comes along with multiple strings is often floor space vs stacking batteries, and the total cable lengths for interconnecting them.

Taller, higher capacity batteries can help with floor area, and interconnecting multiple strings. Higher Capacity batteries can eliminate the need for stacking, with thermal issues that can come up.

A single string is quite nice, although this can come with higher cost to get the required AH Capacity in a single string.

4D batteries are just a bit smaller than are 8Ds.

New2, so the batteries that you have chosen are Flooded Low/No Maintenance Lead Calcium batteries? OR ... ?

Thanks, Vic

new2PV · June 2015

I dont have the room for not stacking them, they are just deep cycle maintenance free batteries designed for a solar application. 7 year warranty 2000 cycles@20% DOD or 1000 cycles at 80% DOD. Not sure of the internal plate structure. Specs below. I can't see that the difference in elevation is going to make A huge difference in the charging profile, I will put the BTS on the top battery, as I assume it would be the warmest. The are 4 Ds, I can;t put more than 2 on a each shelf.
Max charge rate is 15.2 VDC, 55 Amps, Recommended Float Charge Rate is 14.3 VDC, 15 Amps, Reserve Capacity is 430 Minutes @ 25 Amp Draw, 20 Hour Rate is 215,

inetdog · June 2015

new2PV wrote: »

they are just deep cycle maintenance free batteries

You have clarified your earlier comment that they are "sealed but wet cell". Maintenance free batteries are not really sealed (no pressurization during charging) and are really just FLA batteries with extra depth of electrolyte and vent caps that are really hard to open.
You really do want to measure SG, at least occasionally, and you will have to pry the caps off and get in with a hydrometer to do it.
I personally find maintenance free batteries (for cars or for RE) to be a bad idea. Especially if you pay extra for it.

softdown · June 2015

I put four 8Ds on almost identical 4' shelves. You can easily do it with 4Ds. I couldn't hardly imagine trying to do things with two batteries/shelf. Remember that you want cable lengths to be as short as possible.

mike95490 · July 2015

new2PV wrote: »

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You better be darn sure that those shelves will be stable with the weight of the batteries on them, and not prone to collapse or tip over in an earthquake.
And the floor underneath, the high pressure loading may split those bricks after a couple weeks. Maybe a aluminum or steel load spreader plate ?

new2PV · July 2015

They are rated at 800 lb per shelf. I will put metal on the bottom as you suggested and bolt it to the wall.

new2PV · July 2015

Doing more research each battery should be within 5 degree F of each other. I will have to monitor temps very carefully...

48 v battery bank wiring help

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