12VDC Backup supply

[(*steve*)]

A larger battery, possibly something like the one I suggested earlier.

 

[Audioguru]

Hi Auidoguru, I'm trying to get the hang of the calculation. 175/40 = 4.375 ~ 4.4mA. Why is 40 used as the divider?

Look at Energizer's Ni-MH Battery Applications Manual on their website. It says to have a trickle charge current that is 1/40th of its capacity or less so that the extreme pressure caused by overcharging does not cause the battery to fart and release its goodies. Its life is shortened if it releases its goodies.

 

[WHONOES]

KISS Keep It Simple Stupid. Don't over think it. Keep the charge rate at a sensible level. Enough to run your device on standby and then have enough left over to charge you battery taking in to account self discharge.

 

[partyanimallighting]

Hi guys, I have the GPS connected to a small 12V power supply at present for testing so i connected the positive line between my meter leads and checked for current draw. Without me dialing into the GPS from my phone, the current draw is between 1.5mA ~ 5mA. When I dial into the GPS and my phone picks up, the built in mic in the GPS connects and the current draw fluctuates from around 27.5mA up to 56.7mA. Is this information useful? I also found an unused rechargeable battery in a parts bin and it's 12VDC 3000mA. It's charging right now and if it's OK, I'd like to use it. It's small, neat and it'll be really easy to conceal.

 

[(*steve*)]

What chemistry is the battery?

 

[partyanimallighting]

My bad. It's actually 1300mAh. It's labelled 12VDC mAh Ni-Mh.

 

[(*steve*)]

The problem will be charging it.

I bet you have good cell coverage where you are. Try fitting the module in the door and take it to a location with marginal cell coverage to get a closer to worst case estimate.

 

[hevans1944]

So, are we all done re-inventing LoJack?

 

[Audioguru]

The Ni-MH battery shown in post #24 has 10 cells averaging 1.2V each and will be fully charged at 14V to 15V.

 

[partyanimallighting]

Love hevans1944's comment!! Based on this 1300mAh Ni-MH battery's amperage, what size resistor should I use? Based on your formula, h/ m (hours to charge / 1300mAh), how do I figure out how many hours for this battery to fully charge? Remember although it's new, it was in a spare parts bin and I have no documentation.

 

[Audioguru]

Calculate the resistor so that the battery has a trickle charge current not higher than 1300mAh/40= 32.5mA.
But the battery will be 14V to 15V when fully charged so it will never be fully charged and it will never have a trickle charge. You can charge it through a resistor if the charging voltage feeding the resistor is about 18V or more.

You can charge a 9.6V Ni-MH battery (8 cells) from 13.8V because it will be 11.2V to 12.0V when fully charged.