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I've come into an electric bike that currently has a NiMH, 24v, 20cell battery pack. On the pack as well it says 8000mah. The previuos owner has misplaced the original charger :(

The bike has been unused for over a year so would it be safe to assume the batteries are dead or going and won't last long? They were probably never discharged before going into storage in a tropical climate. If I hook it up to a multimeter, what voltage would mean empty and what would mean full? What settings should the multimeter be set to?

Main question is, can I create my own battery pack that uses say only 15 cells max (so I can use my current NiMH charger/discharger) that can give me the same if not a bit more power? I assume I can't test or fully charge/cycle the current 24v/20cell battery on a NiMH charger/discharger rated for only 1-15 cells?

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    \$\begingroup\$ 15 cells will only generate 18V fully charged. A better bet would be to build two 10-cell packs. Charge them separately then connect them in series to power the bike. \$\endgroup\$ – DoxyLover Jan 22 '15 at 5:31
  • \$\begingroup\$ That's what confuses me. I know nothing really about batteries, but I would think that a AAA nimh battery would get no where near a D size battery. But they all seem to be 1.2v? So 20AAA = 20D? \$\endgroup\$ – garethb Jan 22 '15 at 5:41
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    \$\begingroup\$ No, they are not equivalent. They both give the same voltage but the AAA gives much less current. Wattage (power) = voltage times current. That's why bright (non LED) flashlights use bigger batteries; AAAs would discharge very fast. \$\endgroup\$ – DoxyLover Jan 22 '15 at 12:00
  • \$\begingroup\$ The original 20cell battery is meant to get 40km on a full charge. If I drop down to 15cells but higher mAH per cell, will the bike still feel the same (power wise)? But would only last maybe 30km? That would be sufficient as work is only 10km away and would save the hassle of disconnecting and reconnecting every time I want to charge! \$\endgroup\$ – garethb Jan 22 '15 at 23:22
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    \$\begingroup\$ The bike's electronics is likely to see the lower voltage as a heavily discharged battery pack and refuse to operate to protect the pack from damage. Even if not and the motor will run at the lower voltage, it will be drawing more current and it or the wiring may overheat. Without knowing details of the bike's electronics, I really cannot be sure. You can try it but you are risking damaging the bike. \$\endgroup\$ – DoxyLover Jan 23 '15 at 0:12
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A 15-cell NiMH pack will give 18 volts fully charged, instead of 24 volts. The bike's electronics is likely to see the lower voltage as a heavily discharged battery pack and refuse to operate to protect the pack from damage. Even if not and the motor will run at the lower voltage, it will be drawing more current and it or the wiring may overheat. Without knowing details of the bike's electronics, I really cannot be sure. You can try it but you are risking damaging the bike.

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  • \$\begingroup\$ This is sound advice. Even if the bike made use of a DC/DC converter to control voltage/speed it would still need to draw more current from the lower voltage pack than with the original set-up. \$\endgroup\$ – KalleMP Sep 15 at 20:01

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