Our deep discharge batteries for marine and RV use, work with progressive dynamics. Since the factory wires them in series, each cell adds its contribution to the total voltage. When one eventually fails, perhaps owing to sulfation, this voltage drops. The battery industry does not replace individual lead-acid cells, because these would be out of balance with the older ones. The logic against mixing old and new batteries is similar.
Why Mixing Old and New Batteries is a Bad Idea
Let’s assume we are doing a barbecue in the garden in the dark. We need an electric torch to see how the food is doing, but find the three batteries in it are almost flat.
We borrow two from the bathroom radio and replace two in the torch. The light is brighter and we can soon enjoy our meal cooked to perfection. The next time we use the torch, it is back to its old habits.
Mixing old and new batteries has let us down. That’s because the two batteries from the bathroom radio have been working harder than the manufacturer intended. Moreover, they have been forcing more electricity through the weak one than they should. This has caused the weak one to overheat. It could even leak and ruin our torch.
The Chemistry behind the Overheating Problem
Batteries are chemical reactions producing constant voltage. The speed at which this happens depends on their internal resistance. A fresh battery has a low resistance, but this increases as it wears out. Joule’s law dictates that thermal energy gain accompanies loss of electrical energy.
Thus, mixing old and new batteries forces an excessive amount of energy through an abnormally high resistance. The new batteries run out sooner, while the old battery overheats. By mixing old and new, we are wasting good money. This applies to mixing older and newer fresh batteries to a lesser extent.
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Preview Image: Battery Mix
1 Comment
Isn’t there a switch that isolates the two batteries? So you can use them both individually on the same circuit without this problem?