When putting in more than one battery in a device, why should they all be at the same charge?
In: Chemistry
If they’re lithium they need to work in their range, you can’t totally flatten them. Different charges can cause inbalance
Batteries of different charges will try to equalize. If they are all relatively the same charge they would equalize without much trouble no matter the battery type. However if there is a large difference then the fully charged battery or batteries will try to charge the used ones. Most non-rechargeables can handle a small amount of charging without issue but larger amounts can cause issues from the battery simply not working up to fires. Most of the time it will be safe but its not worth risking a fire or device damage to save a few bucks.
As batteries lose charge, their voltage drops. If they all discharge at about the same rate, that’s totally fine. But if you were to put a fresh one in, all the batteries would rapidly balance charge. And the old ones can’t take that sudden recharge
I assume you’re talking about a device without an internal battery management unit that controls the charge and discharge rate of each cell and distributes the load as evenly as possible.
Let’s say that a device requires X volts and Y amps and it carries two battery cells. One cell has half the energy of the other, and in spite of the discrepancy, both still maintain a similar load sent to the device. Without a battery management system once the weaker cell has no more energy or it goes under a certain value, the device stops working or its functions are affected, there might be another option which is to demand more load out of the remaining cell, the demanded load might be more than the cell was designed to send to the device and cause a burnout or short circuit or make the cell unstable.
This is why basic devices come with instructions to replace all batteries at once for new ones, preferably from the same box as those will supposedly be built and shipped with the closest possible specifications.
Short version, the more charged battery will drain into the less charged battery not the thing you want powered
Depends on how the multiple batteries are connected together.
For parallel connections, batteries with a higher state of charge will be transferring their energy to the batteries with the lower. Non rechargeable batteries cannot be recharged and having energy pumped into them will likely cause them to leak as a result, while the rate of power transfer for rechargeable batteries might be too great and cause damage or overheating.
For series connections, the device will work for a while until the weakest batteries are discharged fully, but because there is still some energy in the others the device might continue working. This will cause the weakest batteries to discharge down to zero and then be charged with reverse polarity. Same as before, non rechargeable batteries will leak while rechargeable batteries will be destroyed.
Most of the comments here are concerning parallel connected batteries, but that’s a very uncommon configuration. Most alkaline batteries are connected in series. When you put batteries in series, a partially discharged battery will become empty a lot sooner than the other batteries. An empty battery can’t stop the circuit from working, though. Instead, the empty battery will get “charged backwards”. That battery will start getting warm inside, and then start leaking. It will eventually ruin the battery contacts in the device.
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