Combining different Amp hour batteries

[Kaliona]

Just purchased a 12V deep cycle battery to replace one that died. The other one is still strong and rated 75Ah. The new one I will be hooking up in parallel with the 75Ah battery is rated at 101Ah. Is it ok to hook up two batteries of different Ah ratings? By my 3rd grade math calculations I will have a total capacity of 176Ah, which gives me 88Ah usable (50%). Or will I do damage to the batteries and/or trailer by doing this? They are both wet cell batteries.

[Grumpy]

You can degrade the lower capacity battery since it will overcharge as it charges faster.

[CAG]

They are going to try to "equalize" and that, as Grumpy says, will not be a good thing. Best replace the other at this point.

[craigav]

Just from the electrical standpoint specific to the question "Is it ok to hook up two batteries of different Ah ratings?" The answer is yes it is ok when combining two or more batteries in parallel but not when in series. Two 12v batteries configured for 12v usage are wired in parallel and with that the total ampere hour rating is cumulative.

What to watch out for though, as mentioned already, the batteries all tend to equalize (in voltage) with each other, so if your older battery goes bad (say for example an internal short in the battery which is common), the new battery(s) would get pulled much further down than you may realize and depending on when you first find out that all your power was really coming from the new battery and the old battery is the cause, that time could really shorten the life of the new battery.

However, with that said, if the older battery is still not all that old and holds a charge well, then there is no reason not to use it, what I would do though is take the time to test it every so often to prevent the issue mentioned above and plan replace it when it shows the first signs if going out on you. Also make sure they are both the same type of battery (lead-acid is the most common type).

I will add that many people like to replace them in pairs and in fact that is what I do for my Diesel pickup as when one goes out the other can't be far behind.

~CA

[salsa]

I use a 75 ah and a 96 ah batteries. I do not have them powered up at the same time. I have a on/off switch on the + terminal of each battery. I only uses one battery at time. When two battery are connected to gather the combination is only as good as the weakest battery. I have had a poor battery pull down a good battery. By using a switch on or off I always have a good battery in reserve. I dry camp in the mountains of Idaho where the night are cold. I like having a battery in reserve.

[Kaliona]

Sounds like it's ok to charge them in parallel, but I should isolate the batteries with a couple switches when under load like Salsa does so you're only drawing from one battery at a time. Correct?

[salsa]

I charge the batteries one at time. I use one battery for 24 hours which is about 20 to 30 amp hours. I charge the depleted battery and switch it off that is now my standby battery.

[craigav]

Originally Posted by Kaliona

Sounds like it's ok to charge them in parallel, but I should isolate the batteries with a couple switches when under load like Salsa does so you're only drawing from one battery at a time. Correct?

Isolating them is not a requirement, but does let you know when one battery fails to hold a load and in doing so also allows you to keep a spare hot and ready to go.

Assuming two good lead acid batteries, it is better to keep them both online to distribute the load. For example, instead of running one battery down 50% and then recharging it while running the second battery down 50% and switching back and forth, you could have them both online and run the pair down 25% (75% left in the batteries) and then recharge. The less depth of discharge between recharging allows for a longer battery life overall. It is ok to charge them both together while in parallel and it is also ok to charge them one at a time if that is your preference.

As a thought, consider this like your two propane tanks (assuming that you have two) you could keep both valves open and if you do and you run out then you are totally out, however if you only use one at a time, then when the first goes empty you are on notice that you have used half of your propane, which is good to know. With batteries though you could use a voltage meter to get an idea of how much charge they have in them, which is easier than estimating how much propane you have left.

~CA

[DNelson]

I use the one lead acid that came with our 2015 RV and when not fishing I add a deep cycle trolling motor battery. I use a switch that has battery 1, battery 2 or both options. Because of the dramatic difference in the two batteries I use only the battery 1 or battery two positions. Works well for me.