connecting batteries in parallel

[auger394]

I have two 12volt deep cycle batteries. They are about the same age, but different brands and a little bit different size. Would connecting these two in parallel more efficient than running off one battery at a time? Or would I be better off running off one at a time? I realize the smaller battery will be drawing from the bigger one a bit, but seems like you may still get longer life by running in parallel rather than separate?

any advice?

[Chad Holst]

i have been trying to figure that exact same thing out..not with a smaller/bigger batt, but if it will increase amp hours. i have been assured by a local electrician that it will infact add the amp hours together to make a longer life.

if you had 2 125 amp hour batteries, and ran them in parallel, you would have 250 amp hours at 12v, minus a little lose.

[outkast7222]

Do not mix batteries of different size and age! If you do you will greatly reduce the life of the batteries. One battery will always have a slightly higher or lower nominal voltage and will pull the other down.

[rambryj]

Do not mix batteries of different size and age! If you do you will greatly reduce the life of the batteries. One battery will always have a slightly higher or lower nominal voltage and will pull the other down.

This is true, it will not be dangerous, but it will not be an improvement over using them one at a time.

If you have two batteries of similar size and age then running them in parallel will increase the capacity. Be careful as to not put them in series as that will give a potential of 24 volts.

[CAMAN]

Even if you are using the same age and size battery also remember if one battery suddenly goes bad and you don't catch it in time it will kill the good one and you will have two worthless batteries. If it is possible I would add a selector switch.

[aquafang]

In theory putting batteries of the same type will double the amp hours. What happens is if you are drawing 4 amps from your batteries the current will split and have 2 ampsnof draw through each. But i like to just run one at a time and have one to take home and charge. I dont run a noisy genny. Have a honda 2000 but still too loud fir fishing.

[Woody Erickson]

I agree that if you want to connect up 2 batteries in parallel, the batteries should both be new and the same battery (ie - same part number). This is how I deal with the batteries in my truck.

If you do want to use the batteries you already have, you should use a battery selector or a more economical solution, a simple SPDT (Single Pole, Double Throw) switch.

1. Connect the negative terminals of the batteries and electronics together.

2. Connect the positive terminals of the batteries to the outer connectors of the switch.

3. Connect the positive terminal of the electronics to the center connector of the switch.

[pheasantpete]

If you want to run two batteries in parallel you should always disconnect the two from each other when they are not in use especially for extended periods. This prevents one from possibly discharging the other which leaves you with 2 bad batteries instead of one. Due to incresed resistance in older batteries and possible internal shorting one may self-discharge faster than the other.It is best to buy 2 new(manufactured in the last 2 months) batteries of the exact same size and brand and run them together. You only increase total power output relative to the current draw. If you have a high current draw like a trolling motor you may see 20% more output because you cut the amps in half to each battery which reduces heating up the battery which is wasted energy. You are also less likely to drain the battery as far down which always shortens the number of cycles lead acid batteries can go through. If you are using a low current draw you will not see much of a gain in total output but if you draw batteries down deep you shorten their service life. So having 2 down say 25% is better than 1 down 50% as far as service life is concerned. But if you have to transport batteries to recharge you now have 2 to take back instead of possibly 1 on a short outing. I regularly get about 6 to 8 years on my deep cycles but I charge them right away, run them in parallel, do not take them way down, and maintain them when they are not in use (put on automatic charger at least once every 2 months). Do not leave them on a dumb 2 amp manual for months because it will slowly stir the electrolyte and you will end up with dry cells. Seen more batteries wrecked that way by guys who "knew what they were doing." My background: avionics, advanced avionics course(electronic engineering), radar technician, reciprocating and jet engine mechanic, airframe mechanic and a boat mechanic for 30 years as a side business and many hours spent studying battery designs. If you want to learn about real deep cycle batteries which are not in your boat go to a solar power HSOforum. I do not post here much even though I have been here 5 years so if you want to argue find another tree. Otherwise if you have sincere questions spend some time with google and do your homework. I am no expert but it would be fair to say I know more about batteries than the other 98% maybe even 99% of people out there. The guys who know more and who are the experts are the guys who design them for a living and you can call Penn batteries or even stop by your local Interstate dealer. They are the one of the few battery sales guys I have met that truly know what they are doing. I also will attest to the quality of Interstate batteries and any other battery brands made by Penn batteries of which some are sold as Fleet Farm Brands. Ask because it may have changed. Some Walmart Deep Cycle batteries were made by Penn back in 2008 and they both still work fine. But their sales guy will not have a clue and they do not seem to be Penn now. Lowest bidder thing. Make sure you check the date code on the battery which is usually alpha for the month and numeric for the year. Ie. A3 which would be Jan 2013. I have seen 2.5 year old batteries sitting on the shelf of a well known national Auto parts store that when tested read very low voltage. Hope that helps those needing new deep cycle batteries.