Subject:
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Automatic RCX Battery Recharging
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Newsgroups:
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lugnet.robotics.rcx
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Date:
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Mon, 23 Sep 2002 15:40:34 GMT
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Viewed:
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2543 times
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Some of us on the leJOS mailing list were discussing auto-battery recharging
with the RCX. Here's some technical thoughts from Philipe Hurbain that might
be useful to others.
p.s. If anyone has anything to add to this please do.
-*-*-*-*-*-
I think that the solution may be rather simple if you can accept a few
drawbacks.
As all batteries are serially connected, fast charging methods must be
avoided, so you have to use slow charge. A standard rate of charge for this
kind of batteries (NiCd or NiMh) is C/10 where C is battery nominal
capacity. That means that for 1.5 Ah batteries rating, you should charge
them with a 0.15 A current (during 12 hours instead of 10 to compensate for
charging losses).
Note that nominal voltage of NiCd or NiMh is 1.2V per element, giving 7.2V
for the pack, lower than the nominal 9V of RCX.
What I suggest you is to get a regulated 9V power supply (that can be
applied directly to RCX since this is voltage for 6 alcaline batteries), and
mount a 1 Watt resistor between it's output and the RCX battery pack. The
value for this resistor (in ohm) will be (9 - 7.2) / (Icharge + Ircx), where
Icharge=C/10 (0.15 A in my previous example, Ircx the current consumed by
quiet RCX while waiting for full charge).
Ircx should have to be measured (on an idle RCX), but it is probably
negligible compared to Icharge, so I suggest you assume it to be 0.
In my example, resistor value should then be 12 ohms.
Note that in that process, the RCX remains powered, so there is no risk to
lose program.
I leave you with the design of the charging connector, for example 2 copper
plates on RCX and 2 spring loaded pins on wall charger (I suggest to place
them on a vertical line, so that robot positionning errors should not result
in a short-circuit). Be careful of polarity too !
Now the drawbacks:
- Charge is slow, 12 hours for full charge
- Battery life might be shortened, C/10 during 12 hours is fine with fully
discharged batteries, but you may be overcharging if they were not
exhausted.
- Charging serially connected batteries is not very good (some get less
charge than others, and in the long run, they will get reversed biased
during use and rapidly destroyed). I suggest removing batteries from time to
time, discharge then recharge them on a standard battery charger)
Cheers,
Philo
http://www.philohome.com
Good luck for the program, IMHO it is by far the hardest part of all this
!!!
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Message has 2 Replies:
 | | Re: Automatic RCX Battery Recharging
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| (...) Yes Brian, and read my P.S. ;-) (...) This is not necessairly true. If you use the right charge method, load time less then 15 minutes, without degrading the batteries (even with positiv effect on the lifetime/capacity) is very well possible (...) (22 years ago, 23-Sep-02, to lugnet.robotics.rcx)
| | | Re: Automatic RCX Battery Recharging
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| In lugnet.robotics.rcx, Brian Bagnall writes: <snip> Are you are talking about chagring batteries while they are in the RCX? If so, what about the heat. When the NiMh batteries are chargin in a wall-wart charger they get quite warm, wouldn't you be (...) (22 years ago, 24-Sep-02, to lugnet.robotics.rcx)
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