One last 24v question regarding batteries

[doc]

We currently have 2 different size trolling batteries in the boat, would this be an issue when we connect them to run 24 v

[bamabassman]

I don't think it will, but i would get 2 exactly alike. Call me superstitious, lol.

[nofearengineer]

When you say different size, how different? I am of the school of thought that says as long as you don't totally discharge them, you will probably be fine as long as they are close in size.

[doc]

both are deep cycle marine one is 600 cranking amps the other is 675 cranking amps

[nofearengineer]

Doc, the cranking amp rating of the battery doesn't necessarily matter in this case. That's just the maximum amps the battery can produce at any given moment on a full charge. Amp-hours, on the other hand, is what you really care about for a trolling motor battery. Also, general condition of the battery is important. Both batteries could be rated for 120 amp-hours, but one could be older and not in as good condition, and only be capable of 90...just an example.

[doc]

Originally Posted by nofearengineer

Doc, the cranking amp rating of the battery doesn't necessarily matter in this case. That's just the maximum amps the battery can produce at any given moment on a full charge. Amp-hours, on the other hand, is what you really care about for a trolling motor battery. Also, general condition of the battery is important. Both batteries could be rated for 120 amp-hours, but one could be older and not in as good condition, and only be capable of 90...just an example.

Both are in good shape, just different amp sises. We plan on replacing them as soon as we sell the old motor, but is it ok to run them untill then in the mean time?

[BeaverIslander]

I'd say so. I did the same thing for a month.

[lowrider]

Gonna be installing the new troller tomorrow. I'll
post pics when I get it done. Darn boat monkey!!

[1FASTLASER]

Hey DOC AND LOWRIDER you ole dogs....how are ya.......Ole Nofear gets the cookie on this issue.......cranking amps dont matter amp hours does........you can run the 2 just be aware that one will last more than the other which may or may not be an issue ....depends on the condition of the 2 batteries........If one is weaker than the other it will pull more amps from the stronger of the 2 and therefore drop your serviceable use time down.

[nofearengineer]

Doc...if you really want to be sure about these two batteries, take a multi-meter, set it to ohms, and measure the internal resistance of each battery across the terminals. They will be very low, so you'll probably want to measure mili-ohms. Ideally, they should be identical.

Since in a 24V series connection, the current through each battery will be the same. However, the voltage drop will not be the same...if the resistance of one battery is greater, its voltage will rise or drop faster than the other. So in an extreme example, at the end of the day you could still read 23V across your trolling motor (which will still run it), but the battery with less resistance could still have 12.5V (still pretty well charged), and the battery with more resistance could read 10.5V (potentially damaging to it). They're probably much closer in resistance than this example, but you get the idea.

This is why you charge 12V batteries in series with its own 12V charger bank, instead of a single 24V charger bank...this difference in charge would keep getting larger and larger, every cycle, until one of the batteries was killed.

In fact, this is why 12V batteries need to be "equalized" once in a while. Internally, they are nothing more than six 2V batteries, connected in series. So the tiny differences in cells get amplified each time you cycle the battery, until one cell is dead while the others are fully charged. Since they're all in series...if one cell is dead, the whole battery is useless.