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Topic: Charger timer (Read 268 times) previous topic - next topic



I have a Midland G8 walkie-talkie with desktop charger. Unfortunately it has no circuitry to turn charging off, so one has to - according to manual - charge batteries no longer than 8 hours.

So I want to use attiny and make timer to cut off power supply after 8 hours. Because for such a long period I need external crystal to measure time accurately, I cannot put attiny in sleep and save batteries.
The question is: can I use the charger to supply power to attiny? Attiny consumes ca. 5mA - does it influence charging process?

G8 battery pack is Ni-MH, 4.8V 800mA, 3.840Wh. Charger: 9V, 300mA, 2.7VA



No, you don't need a crystal. You don't care if it's 8.01 hours or 7.99 hours do you?

The charger won't notice the extra load of the Attiny.

If you knew what type of battery (NiMh, NiCd...) then you could put in a proper battery management system to measure voltage and temperature of the battery.
GoForSmoke: "What GShield? You never mentioned a shield."


No, you don't need a crystal. You don't care if it's 8.01 hours or 7.99 hours do you?
I did some tests and it is much more than that...   Furthermore accuracy depends on power source - I tried 5V, 3.3V and 2 AAA batteries. That's why I decided to use external crystal (I really don't want to damage  the radio).

Battery type is Ni-MH, 4.8V 800mA (link)


Apr 15, 2015, 11:00 pm Last Edit: Apr 15, 2015, 11:17 pm by Wawa
Accurate charging time for NiMH batteries is irrelivant. The advice in the manual is not entirely correct.
That charger might fully charge a flat set of batteries in 8 hours. In that case a timer will work.
But if you charge a partially drained set for 8 hours, it can age the batteries.
If you use a timer, you still have to use your brain.
Better NiMH chargers use delta-v detection. They actually know when a battery is full.
Why don't you use a simple plug-in mechanical timer.


The following is what the manual actually states :

Battery recharge
The supplied battery pack is NiMH type 4.8V 800mAh and must be recharged only when it's inserted in the radio;
it takes 8 hours to fully recharge.
To recharge the battery pack:
Connect the socket of the wall adaptor to a mains power socket and insert the jack of the wall adaptor into the
desktop charger plug.
Place your transceiver into the cradle of the desktop charger. The red led of the charger will light up.
When charging is complete take the transceiver out of the cradle and detach the socket of the wall adaptor from
the mains.
! Do not overcharge the batteries! When these are fully charged the charging process does not stop
automatically. Do not forget therefore, to remove the transceiver from the charger as soon as the
batteries are charged, otherwise the radio and batteries may be damaged.
! Do not try to charge alkaline batteries or non rechargeable batteries. Make sure that when you charge
the radio, only rechargeable NI-MH batteries or the supplied battery pack should be contained in the
battery compartment! Alkaline batteries are not rechargeable! Batteries which are not suitable to be
recharged may leak, explode or even burn and cause damage!
! Using a different battery charger other than the one specified can cause damage to your device or may
even cause explosions and personal injuries.

Basically it says it takes 8 hours to recharge flat batteries (not the same as what you state) and that batteries should be taken off charge when fully charged.  You can ignore the need for an 8 hour timer as it won't satisfy the rather vague warning about overcharging.


That's exactly what I said.
Only charge flat batteries for 8hours.
Reduce charging time if you just want to top up.
Unlike NiCd, NiMH batteries have no "memory effect". Partially charging them is ok. Overcharging is not.
NiCd batteries can handle overcharging much better than NiMH batteries.
Plenty of info about that on the web.


Well, it seems that I missed the most important point and I'm overcharging batteries every time...

Anyway thank you for your help. I need to think this over and change my approach.


Just buy a wall timer and plug the charger into it.


^ Said that in post#3

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