Battery Health ? Discharge, Charging Capacity

rtribble · March 2014

So I have observed the following:
1. batteries are charged to there full state (ie. 57.6 V for 120 minutes) , and are then in float mode
2. The battery voltage drops to 51 V within 10 minutes ?
3. The battery bank is nominally 12 X 230 ah at 48V = 2760 AH at the 20hr discharge rate,
4. but discharge rate is quicker so maybe the real calc needs to be at the 6 hour (180ah) rate or 2160 ah capacity

Questions
a. is this to be expected ?
b. what should my Charge rate be ?, I have been using a max charge of 200 amps VDC
c. can you calculate the real AH capacity of a battery bank based on AH in /AH out /and starting and ending voltage ??

Cariboocoot · March 2014

Re: Battery Health ? Discharge, Charging Capacity

rtribble wrote: »

So I have observed the following:
1. batteries are charged to there full state (ie. 57.6 V for 120 minutes) , and are then in float mode
2. The battery voltage drops to 51 V within 10 minutes ?
3. The battery bank is nominally 12 X 230 ah at 48V = 2760 AH at the 20hr discharge rate,
4. but discharge rate is quicker so maybe the real calc needs to be at the 6 hour (180ah) rate or 2160 ah capacity

Questions
a. is this to be expected ?
b. what should my Charge rate be ?, I have been using a max charge of 200 amps VDC
c. can you calculate the real AH capacity of a battery bank based on AH in /AH out /and starting and ending voltage ??

Hold the phone. The battery bank is what? 2,760 Amp hours @ 48 Volts? With twelve batteries of what Voltage? If that size is correct 2 hours of Absorb probably isn't enough.

You say your discharge rate is "quicker": how many Amps do you pull?

The symptoms you describe of quick charge and quick discharge indicate severely reduced capacity.

The only thing you can judge from Amp hours in/out is how many Amp hours you are putting in and taking out.

Vic · March 2014

Re: Battery Health ? Discharge, Charging Capacity

Hi again rtribble,

First, believe that your Absorption voltage needs to be higher -- 59.0 or even 60 Volts (at least in the Winter).
Believe that 2 hour Absorb will be too short a time. The Absorb time will vary, depending on the Depth Of Discharge.

The drop in battery voltage depends upon several things. First, the battery voltage will depend upon the loads on them. AND, the battery terminal voltage needs to be Temperature Compensated.

Do you have a Clamp-On DC Ammeter? This will be a very important tool for you to use to help you determine the current balance per string of batteries during charge and discharge.

In theory, your Charge Rate should be in the order of about 10% of the 20 hour Capacity of the battery bank. This could be up to 275 Amps in Bulk. The SW 5548s should have a combined Max continuous charge capability of about 300 Amps DC. This might be beyond the capability of your Generator. AND, if each battery string cannot Accept its EQUAL share of charge current, trying to set the Bulk current at 10% of C for an extended time could possibly damage those strings that are accepting more than their share of battery current (while some of the others are laying on the beach - not accepting much current).

There has been a previous discussion here, regarding the ins and outs of your Massively Parallel String battery bank:
http://forum.solar-electric.com/showthread.php?18915-Batteries-Massive-Parallel-and-Series-Issue-3-questions

Measuring the SG of EACH CELL of your battery bank is a must. You should use a good Glass Hydrometer with a glass float. Having several will help you know how accurate the one that you choose to do this actually is. I do not need to explain to you that this task will require massive effort. You will need to number each battery and perhaps use a letter for each cell. You will want to record these actual measurements in the Battery Logbook. Perhaps you can dictate these readings for each of the 432 battery cells. ****EDIT: This number is WRONG, others note that the # IS 288 ... DUUhhh!

While your batteries are charging at relatively high current, use your DC clamp meter to measure the current in each string of batteries, and record this, too.

More later. We have been a bit rough on you. We all make our best decisions, and do our best to use what we have. Hope that you will continue to hang in with us, as we try to help you work through your issues. Good Luck, Vic

vtmaps · March 2014

Re: Battery Health ? Discharge, Charging Capacity

Vic wrote:

for each of the 432 battery cells

Cariboocoot wrote: »

Hold the phone. The battery bank is what? 2,760 Amp hours @ 48 Volts? With twelve batteries of what Voltage?

Vic, thanks for linking the other thread. If I understand what I read, it's a mere 288 cells.

Cariboocoot, I think the OP has 48 floor scrubber batteries, 12 volts each. They are configured as 12 strings with 4 batteries per string. They are high quality batteries that require long charge times and a daily 62 volt "finish" charge.

Rtribble, there's not much to say that hasn't been said in the other thread, and Vic has summarized the things you need to do in this thread. As mentioned in the other thread, you also need to separate the strings and test them (charge and discharge) one at a time.

Twelve parallel strings can work, but it takes MASSIVE and REGULAR (= never ending) effort to make it work and keep it working. If you are not willing to do what it takes, you might as well write off the bank and start over with something you can manage.

--vtMaps

waynefromnscanada · March 2014

Re: Battery Health ? Discharge, Charging Capacity

12 parallel strings of batteries??? 288 individual cells to regularly check for electrolyte level and SG? I thought constantly keeping an eye on 18 cells in by former bank and trying to make 3 strings work was a pain! OMG!

Cariboocoot · March 2014

Re: Battery Health ? Discharge, Charging Capacity

There is almost no chance that twelve parallel strings would work even on a 48 Volt system.
Every battery is a varying resistance, and so can be every connection and every wire. The chances of that many sharing current equally is nearly nil.

I'm hoping we've still got the battery bank specs wrong. If 2,760 Amp hours @ 48 Volts really is needed (33kW hours @ 25% DOD) this would call for either forklift batteries or individual high-capacity 2 Volt cells. What's more the floor scrubber batteries are as vtMaps said: in desire of very high charging Voltages.

Sure would be something to see the 17+kW array (and four charge controllers) that would be need for such a bank, though.

jcheil · March 2014

Re: Battery Health ? Discharge, Charging Capacity

This is never going to be worth the effort. Several year ago (when I was on a 12v system), I decomissioned a huge UPS and the batteries were only about 2 years old so I figured, since they were free, and my existing T105's were 6 years old and at the end of their life, I would just use these 48 12-volt batteries as an experiment to get me by for as long as I could.

Well, long story short, after lugging 3000lbs of those batteries to my site, and spending several hundred dollars on interconnect cables, I was constantly having issues with keeping them balanced. To make matters worse, they were AGM (sealed) so I could never check SG. I had a thermal runaway issue 2 times, lucky that I caught it before it started a fire. No matter how I wired them, they NEVER balanced.

I would sometimes have the SAME issue the OP has; bank appears charged, then voltage drops within a couple hours. Load tests on the individual batteries would determine that some had either open or shorted cells once in a while. So had to remove those as I found them. Then things went back to normal; until the next month...rinse and repeat

Other than the cost of the cables, it was a "free" experiment. But in the end, I would NEVER recommend it to anyone. It is simply NOT going to work in the long run. I learned that Free IS worth what you paid for it

NOTE: I used one of those old-school clamp on load testers to determine which of the batteries were the problem. Not sure if it is "right", but it worked for me in the situation I was in.

Vic · March 2014

Re: Battery Health ? Discharge, Charging Capacity

vtmaps wrote: »

Vic, ... If I understand what I read, it's a mere 288 cells.
--vtMaps

Yes, vtmaps, I cannot multiply in my head, and should not even try anymore ... on getting into bed last night, recalled this battery config, and only then realized that my math was wrong, but was too lazy to boot the computer to fix it ... thanks.

This is an unfortunate situation. Do not know what the nature of the loads are that would require a system of this magnitude.

rtribble, will let you digest some of what has been said ... trying to cull the best batteries from the others, and running those in some part of the system, while you try to recover the others by remedial charging and EQing may help some.

You must have some huge loads in a business or perhaps a multi-family residence.
What is the nature of your loads? Can you conserve power demands more than you have already done?

At my part of California, we have gone from wall-to-wall sun for most of the Winter, to day after day of complete cloud cover. The solar contribution has been quite low. Expect that rtribble's situation may be similar, and perhaps having little PV power contribution puts him far behind the curve.

Good Luck, Vic

waynefromnscanada · March 2014

Re: Battery Health ? Discharge, Charging Capacity

Vic wrote: »

I cannot multiply in my head, and should not even try anymore ... Vic

Hahaha And that's why I always keep a calculator beside my lap top, I can't do math in my head either.
Yeah, I know the laptop has a built in calculator, but the little hand held is always ready to go at any instant. Hahaha

BB. · March 2014

Re: Battery Health ? Discharge, Charging Capacity

And my calculator is solar powered.

-Bill

Battery Health ? Discharge, Charging Capacity

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