48V Batt Question/s...

Revolutionary · December 2011

Greetings Folks,
I have a 48 V Battery, it's 1105 AH, (actually (2) 24 V Batteries in series).
My simple question is this-
I discharge the batt down to about 46.5 volts, then recharge it using the grid inverter/charger I have, a Magnum 4448. It charges at a rate of 60 Amps or so per hour. Noticing that today, as an example, it's put in almost 800 AH & still hasn't reached the 58 Volts or so I need to start Absorbing, per Magnum & my battery Guy.
This doesn't make sense, does it take more Amps to charge than to discharge, and by that much? By my calculations, I was at about 65% (left) in the batt when I started charging again. So, I'm thinking I need maybe 35% of 1105, or 375 AH's, right?
Also, when I'm charging I'm not using anything from the batteries, just running on grid power.
Go ahead, tell me I have a awful sick battery.....(HOPING not...)
Thanks kindly-
Chris

waynefromnscanada · December 2011

Re: 48V Batt Question/s...

Two things. Yes, it takes more current to recharge a battery than you take out because batteries are not 100% efficient. There are losses in the chemical processes, resistance losses etc. It can take 10% to 30% or even more extra current to recharge, depending on the battery and it's condition. Generally the closer to end of life, the more it takes to recharge. Also, the faster the battery is discharged, the lower it's efficiency, thus the less power you get before it needs recharging.
Second, if you want your batteries to last, you should never drain them down that low. (46.5 volts) Even though they are (I assume) "deep cycle" lead-acid batteries, they should never be drained like that. It will kill them and take them to an early grave.
Taking them down to 49.7 volts every day is fine, and taking them down to 48.25 is OK just once in a while, but NEVER take them down to 48 volts or lower. Note: These voltages are measured after allowing the battery to rest for at least 3 hours, NOT while the battery is under load, or being charged.
Hope this helps.

niel · December 2011

Re: 48V Batt Question/s...

the type of battery you most likely have is forklift type and they require oodles of current to keep them charged as they aren't very efficient. the 60a the magnum supplies is roughly at a 5.4% rate of charge and i can almost guarantee forklift batteries will want at least double that rate.

5% is a 20hr rate and then it'll take even longer due to inefficiencies all providing that it is charging at your 5.4% rate. self discharge rates are high on this type of battery too.

Revolutionary · December 2011

Re: 48V Batt Question/s...

Wayne, thank you for your help.
I don't get it, I guess.
My batteries are forklift batteries, FLA, and I thought their cycles were based on taking them down to 20%? If what you're saying is true, I can't or shouldn't take the battery down more than 25% from full? 48VDC is 75% on these batteries, 49.7VDC is almost 90%, so according to what you said the most I should draw down is 110AH a day on an 1105 AH battery? That seems like not a lot of power to use, knowing that my house is going to use 300 AH a day @ 50VDC.

Revolutionary · December 2011

Re: 48V Batt Question/s...

Neil, also thank you kindly.
So, your thought is that I just need more current to charge these batteries better, to a better state? I buy that, as in summer my panels really charge well, and the batteries seem to be in a lot better condition. I have 98 First Solar Panels, a 7KW+ array, on my roof, as well as an old (REBUILT!!!) Jacobs Windmill (3KW) in the yard.
The battery I have is a 2008, and was 100% (this summer, late), so it shouldn't be in bad shape....I really but into the idea that charge rate is just too slow, this makes a lot of sense to me.
Just finishing getting my (again rebuilt) 16HP Lister hooked up/finished, in the garage, am hooking it to a 12KW ST AND a MARS Motor, for direct DC charging. (Before anyone freaks out & thinks I'm trying to pull 12KW, I'm not. The MARS will make most of the power, I will adjust charge current on the Magnum to get somewhere in the 6-7KW total range, mostly to batts, but I should be able to get into the 100+Amp range for charging without much difficulty.)

Thanks again-
Chris

niel · December 2011

Re: 48V Batt Question/s...

the amount you discharge the batteries needs to be put back in and thensome due to those inefficiencies. i'm not real sure what efficiency may be on your batteries, but is most likely around 70%. that means it takes at least another 30% to bring it to full.

now without going into details on how much you can use of your batteries' power daily, it stands to reason that it won't be much comparatively for such a large battery bank. if you are relying on the grid to keep your batteries charged i might suggest you buy another charger to supplement the charger in the magnum to allow a better and quicker charge to your batteries, which will also allow you a greater range of use of your batteries on a daily basis. finding high current 48v chargers is difficult as this is a great deal of power. try a forklift company to see what may be available from where and be sure you are aware of the outlet requirements as some may need a 220vac outlet with substantial current abilities. if you can't find anything, do know that our host does carry a 15a 48v iota charger.
http://www.solar-electric.com/iodl48vo15am.html

do note i was composing this while you had posted again. i think you are getting what i'm conveying.

Revolutionary · December 2011

Re: 48V Batt Question/s...

Right on, Neil, this makes all the sense in the world.
I'm still thinkng I musy have a bad cell or seomthing, as I'm lookng at the charge on that batt right now & it's taken 1013AH so far & is in Absorb now, & I had it at 46.5 VDC. Assuming a 30% loss + the charge Amperage itself, this should be a 375AH charge, +30%, right? That's still not much over 500AH, & I'm double that right now, am I seeing something wrong myself, or is this a normal thing for what I'nm dealing with based on a low (5.4%) charge rate?

niel · December 2011

Re: 48V Batt Question/s...

you can check the at rest voltage after 3 or 4 hours to see if the voltages dip due to a bad cell.

like i said it is most likely around 70% and i was guessing. it may be 50% for yours as i don't know for sure.

mike95490 · December 2011

Re: 48V Batt Question/s...

As deep as you have been pulling that battery down, it may take you several cycles to get it back up to full again. Try an extended ABSORB cycle, to where the current starts to drop off quite a bit. (That size battery may not go less than 20A from the charger)

Revolutionary · December 2011

Re: 48V Batt Question/s...

Thanks for the info.
The battery charges back up to absolute full every time, that's not what concerned me. What bothers me is the actual amperage required to get it back to full.
This morning, I checked gravity on the battery, full, and got 1.31 or so across the board, and 2.18 volts (or more) on each cell, after the batt sat for about 3 hours w no load & no charging. Spoke with my battery guy, & he told me that the battery I have is designed to be discharged down to 40.8 Volts, so according to him I'm doing no damage by coming down to 45-46 VDC.
I have another battery that I've had for years & years, but never used it as much. It's an 875 AH Batt, 48V, but again, I haven't used it as much as I am the 1105.
I'll take the battery down again, and check voltage & gravity, & see what happens.
Mike, the absorb on it is set to 5.5 hours right now, per Magnum...
Thanks again for the input-
Chris

Vic · December 2011

Re: 48V Batt Question/s...

Rev,

Couple of things ..

Seems to me, that most of the Traction batteries need a Hot Asorb voltrage setting. Something on the order of 60 volts ... but your batt guy knows best.

And, am sure that you are using BTSs with each charge source (?). In the Winter, batts often get cold. Do not know where you are located, but think you implied that it is NOT Summer where you are. Cold temps mean that compensated battery voltages rise.

1.31 seems a bit overcharged to me. IIRC, most Traction batts are fully charged at about 1.280.

A cold battery needs SG readings compensated lower than float hydrometers read ...

And 70% charge efficiency sure seems LOW to me. Most of that 30% loss would show up as heat in the bank ... Like some here, I am just guessing.

EDIT: Many Traction can take discharge to 20% SOC, BUT, they need to be recharged in 8-12 hours to reduce the liklihood of damage. THis takes a very LARGE charger. And think that your voltage readings at the bottom of discharge are prob not resting voltages, so if they are coltage reading with little or no rest period, should not be a problem at all, even withour considering that this is a traction battj, designed for deep discharges.

Good Luck, Vic

48V Batt Question/s...

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