Roomba 14.4-V Ni-MH batteries

I have been given a 14.4-V, 3800 mAh battery pack of the kind used by Roomba vacuum robots.
It’s new. When I measure the voltage, I get 15.2 V. This would work with my Xiegu G90, for instance. The battery has four terminals: two large terminals for positive and negative, and two small terminals, presumably for some kind of control. It is a bit heavy for SOTA at 665 g, but I am considering the possibility. I am, however, not sure about how to charge it. Has anyone used this kind of batteries?
Thanks!

imatge

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l’m sure there are commercial chargers available. But what has worked well for me is a power supply with voltage and current limiting.

A single NiMH cell has a nominal voltage of 1.2V, and a peak voltage of about 1.44 volts or so at full charge. I count 12 cells in that package, so nominal is 14.4V and peak voltage would be around 17V on the charger. (I have to recheck those, but somewhere around those numbers.) So you want a regulated output of about 17V - 18V (preferably adjustable). A 24V supply with an adjustable regulator might be a good starting point.

Maximum charging current probably shouldn’t exceed 1/5 of the total rating (although some cells are designed for fast charging). So I might choose a current around 500 mA, or even less (250ma?) if I’m not in a hurry. But this needs to be a current regulator with an explicit (adjustable) current limit, not just a supply that isn’t rated for any more than that.

So you hook it up, and it draws 500mA (or whatever) until the voltage is high enough, then the voltage limit takes over, and the cells draw less current until they reach the peak voltage, at which point they stop drawing current. A suitable light bulb in the wire before the regulator give an indicator of how much current it is drawing, or you can put LEDs in the regulator to show which is active at any time.

There are simple circuits and complex ones that can be used, but that should be a reasonable summary.

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NiMH cells need a constant current charger not a fixed voltage supply.

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Constant current for most of the charge, but voltage limited at the end to prevent overcharging. That’s why both voltage and current need to be limited.

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No, no voltage limitation, the Ni-MH battery does not like that.

73, Peter - HB9PJT

A current limited regulated voltage supply is not a constant current source. And NiMH chargers are meant to use constant current till you get a -ve delta V on the cell(s).

Thanks a million, WB6BYU, HB9PJT and MM0FMF for your replies!
@WB6BYU, yes, 12 cells; as the nominal voltage of a NiMH cell is 1.2V, the package reads “14.4 V, 3800 mAh”. I searched for a charger but could not find anything available in Europe. I know I have often read 1.4 V in fully-charged NiMHs; therefore, 17 V sounds about right, which is possibly too high for the Xiegu. I have a regulated power supply, but it only reaches 16 V: it would only bring the batteries up to about 1.33 V.
Cheers!

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Hi Mikel,

I am only answering about the charger and not whether this battery is a good choice for your Xiegu, which is another topic.

There are many such chargers on the European market.
One that is readily available and used by many SOTA activators (including myself) is the SkyRC iMAX B6mini.

You can use this charger for different battery chemistries and charging configurations.

73 Stephan

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Hi Mikel,
There are advantages to not fully charging cells to their maximum voltage. By doing so you extend their actual life and as you want to keep the fully charged pack under 15 volts, I would say charging (through a controller like one of the iMAX range) to perhaps 85% full would bring two advantages - extended life of the battery and not blowing up your G90!

73 Ed.

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I read somewhere a while ago that the G90 can handle 17V input. I use it with a fully charged LiFe PO 4S. I see around 16.8V on the internal Voltmeter when the battery is fully charged and no problem at all.

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Found the article on Reddit: https://www.reddit.com/r/amateurradio/comments/18wsxa3/4s_lipo_on_xiegu_g90/

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I use a bridge rectifier in series with the power lead if my battery is over 16v. there are some losses but I only use it till the voltage drops a little. G90 still goes after 4 years.

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