My wife is handicapped and as a result, every 2 years I get to replace the gell batteries in her power wheelchair (12 V 55 Amp/Hour Deep Charge). While the 2 year old batteries only hold about 80% of their peak capacity after 2 years of use on her power wheelchair, when charged, I can run my 1000 GPH pump for at least 15 hours without any problems. I normally have trouble finding people who want them (I gave away 3 batteries last summer - a year and a half from now, I'll have 2 more that get swapped out). These are not cheap low end batteries (New, the 55 amp/hr batteries are at least $150 each to replace - the 75 amp/hr batteries my wife's power scooter used are over $200 each).
My point here is that if you know someone in a power wheelchair or 3 (or4) wheel electric mobility scooter - they will be replacing batteries every 12 to 24 months. You should ask them for their old batteries - My guess is that you will get them for free. You will need an AGM battery type charger (easy to find) otherwise you will ruin the cells recharging them.
You will find 17 to 100 amp batteries used in electric mobility devices. Some people will have ruined their batteries, but most of the people I know that are in mobility devices take care of their units because they depend on them daily.
To use them, I just clip onto the battery connections. I sometimes run 2 pumps off the same battery - recharging it in the evenings before using it the next day.
