any lead acid battery gurus in here?

Factual Questions
1

I am trying to think of a simple way to charge a sealed lead acid battery in the car for use as a UPS for a 12V system. The battery is a standard 7AH, 12V battery: http://www.power-sonic.com/ps-1270.pdf

Is it possible to just hook this up in parallel with the cigarette socket output and the device to be powered? It seems to me that the 12-14 volts output of the car system would float charge this battery (like the one already under the hood) and then when the power is interrupted by unplugging the cigarette socket, power handoff should be immediate since it is already hooked up across the input to the device.

Since the car voltage will in most cases (I realize it varies) be higher than the battery when the car is running, the device should draw power from the car and not the battery, right?

Any risk of explosion here? Will the battery try to draw too much current when charging? The internal resistance is 22milliohms.

2

A straight Gell Cell, which power sonic seems to be (not AGM) needs to be current limited. A series resistor will be the easiest way to do that. A couple of ohms is enough, but needs to be 5W or so power rating.

3

Hmm… I supposed I should have added that I need the battery to put out about 7-8 amps when it is acting as the UPS. Perhaps I could add a power diode and resistor in series instead so that charging current (inflow into the positive terminal(?)) would be limited, but output current would not. (crude diagram below)

Would a glass mat battery not be as sensitive to charging voltage?

Thanks for the help,
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+
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4

|…|
|…-.|-----------------------
|…|
|…+|-----|<|----///------
|…| 7Ohm/10W
|…+|-----|>|---------------
|_____|

5

AGMs are usually starved electrolyte chemestry (gates patent) so need a little higher charging voltage than gelled or flooded batteries…they will take high charging currents though.

The diode idea should work fine. I’d put just one diode directly in paralell with the resistor.

If you want to isolate battery from car power when ignition off, use a relay controlled by the ignition switch. Using a series diode as you have shown will prevent proper charging, as even a schottkey will have several hundred millivolts of forward drop…and shottkeys have fairly high reverse leakage, so it won’t isolate well anyway.

6

I was looking around online and the max charging voltage for the AGM’s seems to be around 15-16V. Is there any danger to holding a battery at this voltage level even after it has reached full charge?

Also, the resistor in the positive lead - what value should that be? If the car battery sits at 14V and a discharged battery is at 10V or so, do I need a 2ohm power resistor to limit current to 2 amps? I should probably size it so that I get an acceptable trickle charge current when the battery is fully charged.

Thanks.

7

There are a couple issues here:

  1. What is the maximum safe charging current for your battery?

  2. When the battery needs to be recharged, what is the minimum voltage it might be?

8

I looked online and the max charge rate seems to be around C/8 for most AGMs or about an amp for the 7AH battery I want to use. I don’t think it will run to anything below 10 volts because the electronics that are draining the battery will shut off then and also because I don’t want to damage the battery.

9

See, that’s a problem.

If you hook up your lead acid battery to your car’s electrical system, and your battery is around 10 V, there will be a lot of current flowing into your battery from your car’s battery and/or alternator (which will be around 12.6 V when the car is not running, and around 14 V when your car is running). Too much current, probably.

Sounds like you need a charge controller between your lead acid battery and the car’s electrical system.