# need help understanding batteries



## Sparticle (Nov 1, 2004)

We have new 2 volt Trojans, 12 of them. Xantrex 3624 and Outback MX60 and C-40 charge controllers. Array is 948 watts. The solar company that sold me the batteries told me I could run a 5000 BTU air conditioner as long as the Sun is shining. I've done that a couple of times but got scared when I checked the batteries with a multi meter. The level goes down fast. The level goes from 28.2 to 25ish in about 20 minutes. Is this normal/okay? Will this kill my new expensive batteries? Thanks folks.


----------



## wy_white_wolf (Oct 14, 2004)

Would like to know 2 additional things.

1. What is the AH rating of the batteries? (or model number)

2. What is the power draw of the air conditioner? (either watts or Volts x amps)


----------



## trkarl (Dec 15, 2009)

since they are new batteries I would advise you to equalize them real good.
Then label each cell with a number.
Then with a quality hydrometer http://store.solar-electric.com/frhdprhy1.html
measure the specific gravity of each cell and record it.

This will give you a base line to compare to. You will be able to tell if you are under charging your batteries by taking several readings on a pilot cell during the week. 

That way you will be able to make sure your charge parameters are set correctly and will be able to avoid problems. You will know if there are problems if the sg slowly falls all week long.

main thing is to keeps records because batteries are not cheap as you are probably well aware of. 

By the way last summer I ran my 2 ton central ac off my battery bank all night just to see what would happen. It is a 48v 546 amp/hr bank of surrette 6cs17ps batteries which would be about the same capacity as your 24v 1110 amp/hr trojan bank. It pulled 256 amp/hrs off the bank or about a 50% dod. This was confirmed with an sg reading the next morning from several cells which measured right around 1.200

The only way to really tell the state of charge is from a hydrometer. The voltage can fluctuate wildly during the day as you are charging then a cloud comes and you have a load running which then is taking from the bank until the cloud goes away.

Your 5000 btu ac probably doesn't pull more than about 600 watts which would be about 25 amps off your bank which isn't even the 20 hour rate. But best to check it with a kill-a-watt meter to be sure.


----------



## TnAndy (Sep 15, 2005)

Nothing to do with your batteries, but why do you have both an MX-60 and a C-40 charge controller ? The MX-60 will handle 1600 array watts on 24v....or 1200 on 12v......


----------



## Sparticle (Nov 1, 2004)

TnAndy, our solar man had us go this route. We have 3 different types of panels. 4 120 watt, 4 77 watt, and 4 40 watt. Total of 948 watts. He said this was the way to go with these different panels. Is it?


----------



## wy_white_wolf (Oct 14, 2004)

Yes, that is the proper/safe way to use panels of different wattages. If the panels have the ~same VMP and VOC they can be used on the same controller without a large loss of performance.


----------



## Sparticle (Nov 1, 2004)

Thanks folks. I'll get a hydrometer As soon as I can. I just checked the batteries with a multi-meter now at almost 9pm. It read 24.9. The Sun was out all day and we were not here from noon on. The panels get Sun from 8am to 6:30pm. Should the charge level be higher? Is something wrong?


----------



## wy_white_wolf (Oct 14, 2004)

Fully charged should have been about 25.4 to 25.5. Was there any load on the batteries when you tested them?

Tojan testing methods and voltages are listed here


----------



## Sparticle (Nov 1, 2004)

Our solar man told us that full charge was around 28 and that half charge is around 25. There might have been a small fan on when I tested them last night. I know I have lots to learn about this and someday I will. We're building a house now and things are quite crazy. Thanks.


----------



## wy_white_wolf (Oct 14, 2004)

I'd suggest you spend some time on the trojan site learning how to maintain them.

I'd say your solar man is only giving you 1/2 the information you need. Yes you will get readings in the 28 volt range right after charging. They are not accurate. You need to wait at least 2 hours (preferably 6) after any charging or discharging for batteries to get an accurate assement of battery charge by using voltage. The only way to get an accurate assessment of battery charge without waiting is with a hydrometer. 

A 28 volt reading would be the static charge from charging the battery. The Trojan site states 25.46 volts as fully charged. 9 oclock should have been late enough to get a close reading after charging but could easily have been on the low side because of the fan.

Something you might want to try is get a voltage reading for each battery individually. Any variation over .02v per battery would indicate that equalizing the batteries may be in order. This is a lot like comparing SG of the cells to determine that, not as accurate but quicker/easier to do.

WWW


----------



## Sparticle (Nov 1, 2004)

Wondering about the hydrometers. Autozone has $4 job, seems cheap to me. Is it sufficient? 

The C-40's are showing that the batteries are fully charged, the green light is solid. So, I'm encouraged that they are okay.


----------



## wy_white_wolf (Oct 14, 2004)

Sparticle said:


> Wondering about the hydrometers. Autozone has $4 job, seems cheap to me. Is it sufficient? ....


No. Cheapies usually only state % of charge and not the actual SG number. They are not accurate enough to show slight variations in cells. Fine for automotive batteries but not for quality deep cells.

This should be the minimum. You want one that is capable of reading down to 3 decimals.

http://store.solar-electric.com/br10prbahy.html


----------



## pancho (Oct 23, 2006)

I have worked in power plants that used batteries for control voltage. Batteries have not been replaced in some since they were installed, back in the 1940"


----------



## Sparticle (Nov 1, 2004)

How 'bout this one? Its a lot cheaper, has cheaper shipping, and it goes to 3 decimal points too. Is it okay?


----------



## Sparticle (Nov 1, 2004)

Sorry. 

http://www.handhelditems.com/temperature-compensating-battery-hydrometer-sp101-ezred-p-56648.html


----------



## Jim-mi (May 15, 2002)

Nope . . .Get the one W Wolf linked to. . . .It is better.


----------



## wy_white_wolf (Oct 14, 2004)

Sparticle said:


> How 'bout this one? Its a lot cheaper, has cheaper shipping, and it goes to 3 decimal points too. Is it okay?


I'm not familiar with that one so can't comment.


----------



## Sparticle (Nov 1, 2004)

Trying to save some money here. This goes to 3 decimal points. Why is it not sufficient? Thanks folks.


----------

