Creating power for your home, off grid.
Emphasis on nuts-n-bolts, hands-on projects.
Note that a "load test" is NOT a "capacity test". A load test draws a set amount of amperage for a given period of time while watching that the voltage doesn't drop below a given level. This will root out any DOA batteries, but will not tell you how many ampere-hours you can expect to draw from them. For your purposes, you need to know if they will run a much smaller load for a longer period. There's no good way to do this quickly, and I doubt that the seller will want to set up an experimental load tester just for you.
Here's what you ~can~ look for:
1) Batteries should have a manufacture date on them. Usually, this will be a code stamped into the lead posts. 49 would mean March 2009, 60 would be June 2010, etc. Sometimes the code is more esoteric, in that case you'd need to contact Interstate and find out how they code their batteries. I wouldn't buy any batteries more than a year, or two (at the most) old.
2) Find out what service the batteries were used for and if possible a description of how they were cared for. Overdischarging even a brand new battery will kill it fast, as will discharging them and letting them sit. Not topping up the electrolyte will diminish capacity. Letting them sit and self-discharge and not giving them a freshing charge is bad for them too. Putting batteries into the hands of inexperienced users (like RV'ers) is tantamount to executing them.
3) Ask the seller if you can try to put together a matched set for your use. Best way to do this is to take hydrometer readings on the individual cells and find batteries that match within a point or two.
4) Are they clean on the outside? Posts bright and not covered with corrosion? Electrolyte covering the tops of the plates inside every cell? Batteries sitting at rest all at about the same voltage? Do they look like they've been smacked around?
If you are putting together a first RE system, you may want to use some inexpensive batteries that you can murder with mistakes before purchasing new and better ones, these might be just the ticket. If you only need to move 20-30 ampere-hour through them periodically, it might not matter that they might have diminished capacity. It would be a different matter if you were putting them in a EV and counting on them delivering 40-70Ah every day, day in and day out, you'd want new batteries to ensure that you don't get stranded.
Dead (terminally dead) batteries of this size are worth $8-10 as scrap, so even if you only get a year or two out of them, you'll still recoup some of your investment, and by that time have a much better idea what exactly you need for capacity. This ill allow you to purchase your next set of batteries with more confidence.
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