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Did I make a huge mistake purchasing these?

rmulgr

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Jun 11, 2021
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Hey folks. I am looking to build a battery bank for my RV. I later want to add solar panels and an inverter. I purchased 10 batteries recently at $30 each. They are all used 12V car batteries out of Mercedes Benz vehicles. Built by exide. They are 95-105 AH each and AGM. Did I make a mistake buying these? I recently heard that car batteries (starting batteries) shouldn’t be used for RVs. If they can be used what depth of discharge can I expect? Please advise
 
Welcome to the forum.

Mistake? Maybe. You didn't over-pay.

Starter batteries do not tolerate discharging like deep cycle do. They are designed for high output and shallow discharges.

Considering you're only $300 in, it's not that big a loss, and that's a fair amount of capacity. If you don't over-use it, you won't be discharging too deeply.

If you want to try to use them, I would try to keep it at > 80% SoC.
 
Well, they are less than ideal but you bought 'em cheap. I would try to limit the discharge to 75% SOC and they should give you some decent service.
 
So .... why were the batteries removed from the vehicles? I'd want to do some sort of capacity testing on them to see if they are any good. Otherwise you may just be adding weight to the RV.
 
Those are thin-plate starter agm's. Typically designed for no more than 5% discharge each start. When playing with starter agm's, one usually tries to limit the DOD to no more than 10-15% per cycle. It is what it is. Lots of dead-lead.

If you want to save time, you'll want to invest in a charger, and most importantly a tool to let you know if you are playing with garbage. I kind of went over it here in another thread:


Spend the bucks and save your time. Test charge each one individually. Worst case if all your batteries are bunk, you can put a nice charge on your vehicles.

OR, turn them all in now as a core-exchange on something a bit nicer. :)
 
So .... why were the batteries removed from the vehicles? I'd want to do some sort of capacity testing on them to see if they are any good. Otherwise you may just be adding weight to the RV.
Good point. I bought a load tester and tested them all. They all test good.
 
Those are thin-plate starter agm's. Typically designed for no more than 5% discharge each start. When playing with starter agm's, one usually tries to limit the DOD to no more than 10-15% per cycle. It is what it is. Lots of dead-lead.

If you want to save time, you'll want to invest in a charger, and most importantly a tool to let you know if you are playing with garbage. I kind of went over it here in another thread:


Spend the bucks and save your time. Test charge each one individually. Worst case if all your batteries are bunk, you can put a nice charge on your vehicles.

OR, turn them all in now as a core-exchange on something a bit nicer. :)
Nice advice thanks
 
I was told that all agm batteries are deep cycle. But I don’t think that’s right.
 
I've done it before with starters - good ones - just knowing that I can't take any more than 10-15% of their rated capacity out of them each cycle. So it can be done, but is just not advisable.

The total irony here is that when you draw such a small capacity out of them, they are relatively easy to get recharged via solar.

The load tester will only go so far. Speaking of which, trying to do a typical C/20 capacity test on starters really puts the hurt on them, especially if used, so if you do a capacity test, only take it to no more than 50% DOD and scale your results! Maybe do only a 10-hour test at the C/20 rate, and double the measured capacity. :)

Oh yes, AGM are deep cycle - but there's a big catch! You can't do it too often! Ie, that capability is only there for the off-hand random deep discharge. But on a regular cyclic basis - no, not unless getting only 100 cycles max out of them is ok. $$

So the natural progression is from starters to so-called "dual purpose" or "marine" deep cycle. They are less acidic than starters, but have the same thin plates. So they now may get only 150-200 cycles from deep discharge. Same deal. Ok, maybe at the most a 20% DOD per cycle if you are going to use them for solar.

TRUE deep-cycle batteries have thick plates, and are designed for long-term power draw.

Still, there's something to learn here. An Optimate charger discussed above will help set up those starters the best they can be. Far better than just a simple vehicular load-test, which is no longer your application with these.

If those batts are healthy, and you draw no more than 10-15% from them per cycle, and they get recharged properly, you may be able to sneak in 3 to 4 years of use.
 
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I was told that all agm batteries are deep cycle. But I don’t think that’s right.
You can deep cycle any battery. It's just a matter of how often before it's borked.

Car starter batteries will not last long if deeply discharged. They are severely compromised when voltage is allowed to drop too far. They have a lot of very thin lead plates and different material design designed to enable a very high current flows for very short periods.

Lead acid batteries designed for deeper and more constant discharging and for use in home/RV/marine energy storage applications have much thicker plates, different separator material with different density and chemical composition. To extend life expectancy for these you'd discharge no more than 50% of rated capacity, preferably less. You can take them deeper and they will recover but it will reduce their useable life.

If you are looking for batteries as an an energy storage for general purpose living functions, then these are not really suitable. To maintain enough cycle life means a very limited depth of discharge (5-10%), meaning you need a LOT more of them to get enough useable capacity and that also means a LOT more weight, a LOT more cables and a LOT more room.

Take them to a lead/battery recycler and get some $ back if you can and buy something more suitable.

If it's for infrequent deep discharge backup purposes then sure, no big deal. If it gives you the handful of deep cycles you need then great.
 
That was my major instinct too. But hey, smoke 'em if ya' got 'em. :) At least before core-exchange.

It can be a good and yes even fun experience, as long as one knows up front the limitations, and sometimes the need for an analytical tool like an Optimate 6 to make sure one isn't starting out with total garbage. When one does, nothing is learned to grow upon...
 
I want the most energy dense batteries I can get my hands on for my RV .... These would near the lowest you can go.
 
Correct. But since he's already got them, and is willing to learn, it could help down the road when someone else tries to do the same, and may have no choice in the matter.

Kinda' trying to turn a mistake into a positive. At least temporarily.
 
As said, it should just work, but keep in mind not using too much, only 5-10% of it's rated capacity, since they aren't designed for this kind of usage.

Assuming 100Ah each, 10 pieces, would give you 1000Ah in total. 10% available would give you approx 100Ah useable capacity.

Considering each battery is approx 20kg, you're adding over 200kg (440lb) of weight. If this is still within the specs of the allowed weight limits of the RV, fine, but still, it's huge and heavy, and definitely not the most efficient...

For $500 or so you can make a 280Ah battery using bulk cells, which would give you approx 225Ah useable capacity (assuming 80% discharge), at 22kg or so (48lb), depending on the box you're building... double the capacity, 1/10 the weight.
 
Thanks for the guidance folks. I will sell the car batteries LOL. I just purchased 4 used lead acid GC2 6V batteries. This should get me started. I understand these give me ~50 DOD so my new 12V bank with 450AH I should realistically see 225AH.
 
So I wired up the four 6-volt Interstate GC2-ECL-UTL batteries in series parallel and installed a battery monitor by AiLi. Trying to configure that now. Each battery says it is 225 AH. What is the depth of discharge on a 6 volt lead acid deep cycle golf cart battery? I can't find this on the spec sheet. I am trying to find out what my realistic amp hours will be. Also, how many volts can I discharge them to? 12v? 12.2v? Help!
 
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