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I have a cordless handheld vacuum cleaner (Electrolux Rapid ZB4103) which is powered by 3 rechargeable AA NiMH batteries (1.2V 1300mAh) connected in series and stacked (glued?) in a pyramid to fit within the case. The batteries are typically recharged by placing the vacuum unit on a changing base (base output is 8V 200mA).

The unit is perfectly usable but the batteries are old and losing the ability to hold a charge, rendering the vacuum essentially useless. I would like to simply replace the old batteries with new consumer-grade rechargeable AA NiMH (Panasonic Eneloop 1.2V 1900mAh) ones.

Question 1: Is it correct to assume that all rechargeable AA NiMH batteries charge at the same voltage and current, and that replacing one brand of battery with another should be safe to use with the original charging base?

Question 2: Given heat caused by charging, etc., is it reasonably safe to stack the new batteries by hot-gluing the bodies together?

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    \$\begingroup\$ Safe? Yes. - How long will the batteries survive? Hard to tell. I'd assume the simple charger just constant-current charges at low current indefinitely. Some batteries are fine with 0.1C indefinite charging, others not so much. Hence, the new batteries may or may not suffer shortened service life. \$\endgroup\$
    – JimmyB
    Commented Mar 10, 2017 at 13:47
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    \$\begingroup\$ Charge rate for NiMH is typically based partly on cell capacity, and since you are increasing capacity, it will take longer to charge, but it will probably still work. Charge termination is usually based on either voltage rate of change or temperature rate of change for fast chargers. Slow chargers may simply terminate after a long time. Most likely, everything will be fine. I wouldn't hesitate to do it if I were in your situation. \$\endgroup\$
    – user57037
    Commented Mar 10, 2017 at 17:23
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    \$\begingroup\$ Agree with others that hot-glue is not the best choice. Kapton tape would be better if you have it. One big piece of heat-shrink tubing might work well also. \$\endgroup\$
    – user57037
    Commented Mar 10, 2017 at 17:25

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1) Yes, charging should be safe enough.

2) Slow charging rarely generates enough heat to be troublesome, though relying solely on hot-melt is not a good idea, bundle them with cable ties as well.

Be aware that not all NimH batteries are equal in discharge current ratings. The batteries you are replacing appear to be very low capacity, 1300mAh, which probably means they are optimised for a higher rate of discharge than your typical torch would draw. I recently replaced the battery in a single cell toothbrush, and found the recommended ones were rather lower capacity, and more expensive, than the typical ~2Ah cell.

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    \$\begingroup\$ Eneloops and similar handle easily 4A discharge. Nothing to worry about. He is replacing dated batteries with latest-technology ones. \$\endgroup\$
    – FarO
    Commented Mar 10, 2017 at 14:35
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1) Yes, since the newer cells are of a quality brand and have the same or higher capacity than the old ones, and the nominal voltage is the same. I'd say that's "safe enough". (Take it from a random person on the Internet.)

2) Possibly. Is there a temperature sensor in the circuit somewhere? Holding the cells tightly together with sticky tape or cable ties, and squeezing some hot glue into the gap between them should do the job without thermally insulating the cells too much from the surroundings.

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