Author Topic: Car inverter question?  (Read 391 times)

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Offline stands2reason

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Re: Car inverter question?
« Reply #15 on: November 23, 2017, 04:24:48 AM »
Just out of curiosity and OCD I tried something:

I've got an oldish HP laptop, 19v DC input from a 90W AC adapter. 11v 5Ah battery. Taking off the battery and feeding 13.8v to the DC jack (not the battery terminals) it will NOT boot, just flashes the power led "anxiously" when I press On/Off. Takes about 10mA of the 13.8v. When I clip on the battery, it behaves like there is just the battery.

I've got the schematics of the laptop. Really!, from some pirate archive. There are couple of comparators which seem to rank the incoming voltage. This autonomous circuit accepts the supply if voltage is over 13 (and drops off at 11v) and allows charging if voltage is over 17. There is also a 3rd wire from the AC adapter that carries some kind of analog information as current. I don't fully understand that 3rd wire, and I'm hesitant to experiment with it. Tempted, but....

Sounds like Li-ion (nominal 3.6/3.7 V), three cells in series. 11-13 V would probably be the charging circuit. it is the approximate voltage range of the battery pack, but older Li-ion batteries top off at ~4.15V, whereas 4.33V per cell is typical for the topping-off charge.

If there is no extra connector on the battery, it is presumably "dumb" and the laptop would respond to 12V directly on the connectors; is definitely in the expected range of the battery pack's output. 12V would correspond to a battery around 80% full, but anything above 13V would definitely be overvolting.

Offline stands2reason

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Re: Car inverter question?
« Reply #16 on: November 23, 2017, 04:35:08 AM »
Just curious why they don't put a small meter on inverters you use in your car to see how healthy your battery is?

Older cars had analog battery gauges which I understand are voltmeters. But that doesn't really answer the question. A shallow-cell that's been mis-used will have deposits on its plates. That leads to a reduction in surface area and increased internal resistance, meaning the voltage will plummet harder when there is a very high current drawn from it. If the battery was already in good condition, watching the voltage with a digital multimeter might keep you safe, but the real test is how much the voltage drops when you activate the starter motor.

 

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