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I have an old laptop that uses a SLA Battery and I'm wondering what sort of circuit would be need to convert the charging circuit for NiMH Cells.

Many manufacturers were able to do some time ago but that information has been lost. The Replacement NiMH Battery had two 6v banks and the computer will output 7.5v to the battery when charging. So I assume I'd need two 45C temperature cutoffs (one for each bank) and also a boost converter to a voltage regulator and then a NiMH charging circuit but I can't find a circuit that will allow the same input and output since the battery only has 2 terminals.

NIMH battery

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    \$\begingroup\$ I would say that board down there is the charge (and management) circuit. Got a schematic by any chance? \$\endgroup\$ – Ignacio Vazquez-Abrams Jan 28 '16 at 23:29
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    \$\begingroup\$ I'm sorry, I don't, the battery is on it's way to my house though so I may be able to reverse engineer it... \$\endgroup\$ – Josh Ryan Jan 29 '16 at 2:52
  • \$\begingroup\$ I am still a bit confused. First what is this a picture of? The NiMH battery pack? Second, how is charge controlled with the SLA? Is the charge controller built-in to the pack, or is it on-board the laptop? As a comment, there are two reasonable ways to terminate charge on a NiMH battery: i) detect rapid temperature rise and ii) detect peak voltage. Cutting off charge at a specific temperature such as 45C is not a recommended way of terminating charging for NiMH. \$\endgroup\$ – mkeith Jan 29 '16 at 17:53
  • \$\begingroup\$ The SLA battery does not have a charge controller inside as far as I know. When the sla battery was plugged into the laptop the laptop just outputs 7.5v to it. The picture is of a battery with nimh cells that was made as a replacement. \$\endgroup\$ – Josh Ryan Jan 30 '16 at 16:52
  • \$\begingroup\$ half of 7.5 is is 3.75 - maybe a dual cell Li-po would be a better solution. \$\endgroup\$ – Jasen Feb 7 '16 at 5:33
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I found this charger circuit : Modified googled NiMH charger And modified it so that it should be able to both charge the batteries in parallel and discharge them (minus diode voltage drop) back to the computer when power is disconnected. I would reccomend low-drop (Schotty) diodes to minimize loss. I hope this helps!

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