Deep cycle battery question.

lesnoy · October 2010

Greetings. I am in progress of putting list of equipment for grid tied solar system centered around Xantrex 6048 inverter with battery bank at 48V. As a battery bank I was planning using Trojan T105, which are sold at local distributer at around 130$ in brand new condition straight from Trojan.

But another vendor which specializing in mostly used batteries has pile of T105 at about half the cost. Upon visiting warehouse to take look at batteries they looked brand new, as of no scratches or dents on outer cases or any kind of build up on contact screws. But guy who sells them said its second hand and he does not know if they were used at all or for how long.

I would love to hear some opinions if this is something worth considering. Or is there any definite signs I can look for that will give me better idea on condition or perhaps a test? Any suggestions would be greatly appreciated.

BB. · October 2010

Re: Deep cycle battery question.

Boy, that is a tough question... First, measure the resting voltage--If it is below 80% state of charge--and you have no idea how long it has been there--then the batteries may be sulfated and have less life/capacity because of that.

But, even then, if the batteries are being charged now--again no idea what happened last time.

I guess, if a battery is sulfated, then you will see low specific gravity after they have been charged/equalized.

Does the battery dealer have any test gear to cycle a battery and measure its capacity?

If you had a money back guarantee (use them for 90 days and see how well they work)--maybe they would be worth 50% off list... Otherwise, I am not so sure.

-Bill

mike95490 · October 2010

Re: Deep cycle battery question.

I'd hold out for 20% of list. Lots of bother to haul to site, install, cable up, charge and eq, and then after a week, find out they won't last the night.

I'm on my starter set still, $25 105's used, and I think they were well used, but all I need is for them to run a fridge and a single CFL at night, so I may make it thru the spring.

Wilis · October 2010

Re: Deep cycle battery question.

Look on Trojan's sight and it will tell you how to tell when they were manufactured. That may be of some help.

Gloves · October 2010

Re: Deep cycle battery question.

A couple of months ago I bought 2 new T-105s from a golf cart company for $115 each.

I can't add new batteries to my existing bank because they won't charge the same. I would get some weird situation where newer batteries would try charging the older batteries.

Because of that reason alone, you might be better off getting the new ones.

BB. · October 2010

Re: Deep cycle battery question.

Sort of yes and no... Measure the resting voltage of the old bank and new bank when fully charged---You probably will find that they are within 0.10 volts of each other--That (very roughly) represents about a 10% difference in charge.

Batteries are only within about 10-20% of their specifications any ways (for various reasons). So--by itself, that is not a good reason to "chuck" the bank vs adding a few batteries for additional storage.

The serious issue is adding different batteries in series. If you have a large battery (200 AH) in series with a small battery (50 AH) in series... the small battery will reach 100% discharge before the larger one... So, the useful available AH capacity of a 200/50 AH series string is really only 50 AH. If you draw more than 50 AH, the small battery will actually reverse polarity and begin to "backwards" charge (because of the unmatched string). That will pretty much kill a battery right then and there.

So, if you have a pair of new (matched) 6 volt golf cart batteries and want to add them in parallel with an existing 12 volt battery bank--Why not. If the new batteries are "better" than the existing batteries--the new batteries may cycle deeper than the existing set until they "wear out" to the same specifications as the current bank.

I think this process is not black and white. What we are usually trying to warn against is, as a battery bank fails, start replacing one battery at a time. What can happen is an old battery may fail shorted or open... A shorted battery will discharge the whole bank (causing damage) or prevent charging current to the rest of the string (damaging the rest of the series string batteries).

At some point, you just want to dump the entire bank as most are near the end of life and replace with a new set of matched batteries.

In real life--perhaps 1/2 your bank will truly fail in 5 years, and the rest may actually last 8+ years in light use (Tony/Icarus has done this with an old battery bank). Why did some batteries fail and other last ~2x longer--I have no idea.

Mixing battery types (AGM vs Flooded Cell) is not a good idea either... But people have done that too.

-Bill

James · October 2010

Re: Deep cycle battery question.

I agree with Mike90045,
You should be able to get those batteries for about 20-25% of list. You might check and see what the price per pound is being paid currently for lead batteries at the recycling yard.
you may be able to buy them at about 5% above the rate, if you are picking them up. Remember, that shop most likely needs to pay for fuel/man hours to move those batteries to the recycling yard.
if not to far a drive for you, the savings may be worth it.

Gloves · November 2010

Re: Deep cycle battery question.

OT: Quick question:
The Trojan T-105s are rated at 225 AH.
I put 2 of these in series to equal 12V.
Does that mean my capacity is 225 or 450 AH?

BB. · November 2010

Re: Deep cycle battery question.

225 ah @ 12 volts.

-Bill

niel · November 2010

Re: Deep cycle battery question.

in series the voltages add with the current the same and when in parallel the currents add with the voltage the same. this applies to pvs too.

Deep cycle battery question.

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