Complex (for me) Battery Calculation - Page 1
Forum section: electronics / beginners

author: bullet308
author_rank: Frequent Contributor
timestamp: July 19, 2012, 05:30:05 pm
content: Hi:I have been working on getting a Russian DP-5V military Geiger counter working on something other than the odd 1.5v batteries it is designed to operate on, which resemble three slightly oversized AA units. They also provided an adapter that goes into the battery compartment that uses a simple voltage divider circuit to step down either 12v or 24v to the required voltages. I just checked the power drawn by the unit and its pulling through the adapter with it set to 12v and its using 75mA, 150mA with the two little incandescent bulbs running to illuminate the meter.At the moment, I have it set up to run either off of the 12v power in the car or off of an adapter that holds x8 alkaline batteries through the same voltage divider setup as for the car. The AAs are nominally rated at 1,800-2,600ma/h each. According to an online battery life calculator (which seems to be optimized for much bigger batteries), if we simply add the batteries capacity, we have a number of about 2Ah, yielding about 25 hours without the lights and 12 hours with. The one thing I don't have at the moment is what the unit is drawing without the voltage divider circuit directly from the battery compartment. Need to go home and do that.The questions that go with all of this are a) do my battery life calculations under the conditions stated pass a sanity test, what with 8x AAs in series like that? b) just how badly is the voltage divider eating my lunch on battery life, and c) is it going to be worth my time and energy to develop another portable power solution that does not use the provided voltage divider circuit? 12/24 hour battery life isnt so bad but I could stand longer.TIA:

author: PA0PBZ
author_rank: Super Contributor
timestamp: July 19, 2012, 07:04:34 pm
content: If I understand correctly what you are saying (three slightly oversized AA units, 1.5v each) then the thing is designed to run on 4.5 volts.So if you run it on 12 volts using a voltage divider circuit, you are throwing away 7.5 volts x the current, that's more than you are using (4.5 volts x the current). Looks like you are using like 62.5% of the capacity to warm the environment.

author: Kevin.D
author_rank: Frequent Contributor
timestamp: July 19, 2012, 07:09:30 pm
content: Hi bullet . If all your voltage regulator is ,is a simple resistive voltage divider or linear regulator then all it's is doing when it reduces the voltage to 5V is wasting the extra power has heat . So with your 8 batts in series your just wasting away 7V or 7/12ths of the power pack.Yourwould much better off making a power pack up to the correct voltage 4.5 V and just feeding that to the internal batt contacts .

author: IanB
author_rank: Super Contributor
timestamp: July 20, 2012, 01:54:45 am
content: I'm not clear on your measurements. When you connected 12 V to the adapter, are you saying the adapter pulled 75 mA or 150 mA from the 12 V supply?In that case, if your AA batteries had a capacity of 2000 mAh (not mA/h) then the expected run time would be in the order of 2000/75 = 26 or 2000/150 = 13, either 26 h or 13 h or somewhere in that ball park.But as others have said, the "voltage divider adapter" is designed for an infinite power source like a car, not a limited power source like AA batteries. You should rather make an adapter that connects three 1.5 V batteries directly to the unit so you are not wasting power.

author: bullet308
author_rank: Frequent Contributor
timestamp: July 20, 2012, 06:48:17 pm
content: Yeah, that was in impression I was under, that it was simply going to throw away a bunch of current as heat, and that I could get much better performance going some other way. I dont really care all that much about it when I am powering it from a 12v power point in a vehicle, but when operating on portable battery power, it really matters a lot. I need this thing to run for days if not weeks on internal battery power. I gather that even with eight AAs in series, there would be no issues with internal resistance even at 150 mA (worst-case)? I have every intention of dumping the incandescent lamps for LEDs, so most of that extra 75 mA is going to go away.Just hooked it up to a bench power supply without the voltage divider adapter in place ...180mA with the two incandescent lamps running, but with those light builbs powered off, only 13mA(!) So, it was in fact throwing away a bunch of power, about 80% of it. Now I know where to go with this. Might use three D-cells in holders I have to maximize battery life.BTW, in the previous entry, the 150mA and 75mA readings were through the voltage divider power adapter attachment with the incandescent lamps on and off, respectively.Thanks all for the feedback.