Let's suppose you have 3 different 12V batteries, wired in parallel to supply 12V power to your RV. They can have different capacities on account of size or age, but the same chemistry (e.g. all flooded lead acid or all AGM). Before you start charging, the voltage across each of them is the same–even if one is fully. There's absolutely no reason for an aged battery to be excluded from a battery for electrical reasons–only for space and weight would that make sense. Would a new. It's common in many RVs to make use of pairs of 6V deep cycle batteries wired in series. In a pair of 6V batteries in series, the voltages of each are not guaranteed to be. Ok, now we're talking about something a little different. In the case of a battery shorted cell, you'll be overcharging the other 5 cells in the same battery. They'll continue. One of the easiest ways to test individual battery capacity is with a portable analyzer. If you only have 12V batteries, they're pretty inexpensive, like [amazon_textlink.
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Can I mix old and new batteries?
A. Do not mix old and new batteries. Doing so will reduce overall performance and may cause battery leakage or rupture. We recommend replacing all batteries within a device. Q. Can I mix different battery types? A. No, different batteries are designed for different purposes.
There's absolutely no reason for an aged battery to be excluded from a battery for electrical reasons–only for space and weight would that make sense. Would a new battery have greater capacity than an old one? Sure. But the one you have is free while a new one isn't.
Even rechargeable batteries will not recharge to the same level as new ones. As such, the following guidelines are important: With primary (disposable) batteries – only use batteries of the same brand and age (ideally from the same packet). If this isn't possible, double check the voltages of each unit with a voltmeter.
Do all batteries have the same capacity?
They can have different capacities on account of size or age, but the same chemistry (e.g. all flooded lead acid or all AGM). Before you start charging, the voltage across each of them is the same–even if one is fully charged and the others aren't. Charge will flow from one battery to the other two until they're balanced.
You'll take it out of a safe (for longevity) state of charge range, without the voltage across the series batteries indicating that. When you charge the batteries, the reverse happens–the battery with lower capacity will be charged faster, and can be overcharged.
But your old battery isn't going to ruin the new ones. It's common in many RVs to make use of pairs of 6V deep cycle batteries wired in series. In a pair of 6V batteries in series, the voltages of each are not guaranteed to be the same as they are when wired in parallel.