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care & feeding of deep cycle batteries

I just bought a 12V, 75 A-h deep cycle/RV battery from Wal-Mart ($45 + $9 core fee), with the intention of turning it into a portable "generator" (via 75W 12VDC -> 110VAC inverter) for overnight use, when we go on a long road trip / camping.

I have a 3A load at 12VDC (two CPAP machines at 1.5A each) that I would like to maintain overnight (at least 8 hours), preferably lasting for at least two days before charging is necessary (at a motel, say).

I also bought an el cheapo Black & Decker 2A "charge maintainer" / battery charger for about $17. I assumed that pretty much the only difference between this and a ~6A regular battery charger is the speed it takes to charge a battery. (At 2A, it should take a day and a half to fill up my 75 A-h battery.) Was my assumption correct? The charger box did say it could be used for charging deep-cycle batteries.

Any comments on using a deep cycle battery in this manner?

The best part about shopping at Wal-Mart is the very convenient return policy. I'd tried using a $15 motorcycle battery, hoping it would put out enough juice; alas, after only 7 hours (at 1.5A), my CPAP and inverter shut down from lack of juice. So, to the Wal-Mart return line I went. "Anything wrong with it?" the sales lady asked. "No, it was just too small." End of questioning; a refund was issued (along with the $9 core charge, yay!). So, if I goof, I can always return everything and walk away.

Thanks,

Michael

Reply to
mrdarrett
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Hi Michael You should only ever use the top 50% of the capacity of any lead acid battery. Totally flattening it is very damaging, and you might find you only get a few full discharges. Most apparatus has a cutout when the voltage dips too low to protect the batteries. For the load you are wanting, you should triple the capacity of your battery. Remember to charge as soon as possible after discharge, otherwise irreversible sulfation will rob you of future capacity. You might just get away with buying another 75Ah and using one for each machine. If it fails half way through your third sleep, is it a life threatening matter? jack

Reply to
spamfree

I'd get a little bit better charger too, one that senses the battery voltage, which climbs during charging and then levels off at the point where things should stop. The good chargers sense this point and it helps prevent overcharging, which helps extend battery life. I agree that you need 3-4 times the capacity to avoid a deep discharge too.

Steve Roberts

Reply to
osr

Ok, thanks for the information. I may well just spring for the 110 A- h battery; it was $15 more. Big n heavy, though.

Not really a life threatening manner if a CPAP fails; dad and I just feel a bit more sleepy in the morning, that's all.

Michael

Reply to
mrdarrett

I was going to suggest a bigger (faster) charger. You could reasonably go to 7.5A (~10% of the capacity). Wet cell lead acids do alright with a simple voltage charger and the max current limited by the size of the charger. They actually benefit from a bit of overharging. Stirs up the electrolyte. Be careful that you turn the charger off before you take the clips off the battery, or you might end up with bits of battery embedded in you with a sulfuric acid douch to sterilise the wounds :) jack

Reply to
spamfree

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