D7 battery charging (Was RE: [aprssig] Considering buying a D7 - Some Basic Questions)

Dave Baxter dave at emv.co.uk
Tue Apr 1 05:32:02 EDT 2008

Looking at the schematics of the D7...

D7 Battery charging:

>From the external power/charge jack, if enough volts are present, and a
battery is connected, there is a constant current "Trickle" charge
regulator in the radio itself.  I think it's about 60mA, not a lot.
(two diode volt drop, minus Vbe of Q6, a 2SD1483, across R12, 12 Ohms in
it's emitter, a classic simple constant current source.)  Looking up the
spec on that device, it's only just up to the job in my limited opinion.

There is NO battery full detection, as its such a low current, it's
deemed "Safe" (in a non explosive way, not battery life way) to allow it
to continue to trickle charge a "Full" battery, for ever and a day.

That trickle charge system will work with the radio switched on or off.
BUT...   If the external power source is itself of a limited ability
(the wall wart) then it will be "problematical" to run the radio, even
in RX sometimes, and expect the battery to receive significant charge.
With an effectively unlimited external power source (12V vehicle feed
etc) the radio will work just fine, and of course the battery will
charge too.

(As we all know, there is also no low battery notification, the radio
just dies, or cycles on/off till the battery goes very dead)

Take care with non Kenwood DC power leads, if the switch on the jack (in
the -ve line) is not opened when external power is connected, you *WILL*
boil the battery, and risk a "violent episode"!

There is very simple reverse polarity protection (one in-line diode in
the +ve line) between the charge/power jack and the radio, but NOT
between the battery and radio.   However, reverse polarity could still
hurt Q6 if it's base/emitter junction is damaged when it zenners, maybe
rendering the internal charge regulator defective thereafter.  (A diode
in series with R14 would prevent that)

Fast charging.

That will need direct connection to the battery.  If the packs also have
external connections allowing a "drop in" system, all well and good.  If
not, the pack will need to be clipped into a charger.   I'm fairly sure
I have seen such things, but note that there are no presure relief vents
(that I know of) on the normal Kenwood battery packs.

If it is possible to drop the radio and battery into a desktop fast
charger, take care, as there is no sensing to disconnect the radio from
the charger/battery that I can see in the schematics.  So, take care if
you have a system that takes the battery AND radio at the same time in a
fast charger, if the voltages go a bit high (over 15V) the radio will

None of the above detracts from what is a briliant bit of kit, and a
usefull tool for APRS, so long as you don't want to use KISS mode with
it's internal TNC to run a digi!..   (Lack of internal memory it seems,
on a busy channel, it *Will* lock up at some point.)

Hope some of the above re the battery helps people understand the beast.
If anyone wants the schamatics, let me know off list please.


Dave G0WBX.
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