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How is "cold battery" sensed? I want the charge.

607 Views 21 Replies 6 Participants Last post by  Markgm
I am a new owner of a used 2000 Legacy wagon. I have very interesting living conditions. Living in the car, I use my laptop all day. Sometimes, it's a little cold outside, too. So I break the rules and idle my car from time to time. I have a power bank/inverter that I charge, and the laptop via inverter uses 15-20 watts. Well, this car doesn't give out extra current for much.

When I start the car cold, it gives battery charge current, and I really enjoy using that. It reminds me of the charging of the old days, when the battery was always being charged. Well, I think I want to do this here. I want to charge the battery all of the time. I will use it.

Does my car have a battery temperature sensor, or is it another temp-related sensor somewhere else, that gives me this high cold-start charge? Secondly, I am wondering if there is a way to trick it into always being "cold", so to speak, so that the higher voltage/charge would always be on?

Cheers, Mark
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I have the impression that most, if not all, of the examples of low voltage/low current are based on the engine idling.

The alternator's ability to generate current is dependent on rpm, On a "cold start" the engine idling rpm is usually raised above normal until the engine warms. That, along with the colder alternator temperature (see paragraph below), helps maintain higher alternator output to replenish the battery charge that was lost to start the engine. Once the engine and alternator are warmed up, the idle speed is lower, leading to lower alternator output capacity as this table from the 2000 FSM indicates for the factory-original equipment (left column is MT, right is AT).

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At idle, the alternator is not going to be able to sustain very much beyond the basic power train needs and perhaps some lighting or other accessory.

Also, the alternator's average output voltage is dependent on its temperature (which is a proxy for the temperature of the battery in the engine compartment). On a "cold start", the voltage will be higher; low to mid-14 v is normal. As the temperature of the alternator increases, the average regulated output voltage will decrease, but usually not lower than the upper 13 V range. The temperature control is built into the alternator's regulator itself, and does not use an external sensor.
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The only thing I do not know at this point is how high the voltage goes on a cold start, but I think that is also alternator-controlled.
When the sense line is disconnected, the alternator is unregulated; there's no control. The output voltage might go well over 16 V (I've seen them higher than 20 V), which could cause premature failure of light bulbs and affect the electronic systems. Also, when the alternator voltage is being regulated, the average output voltage will normally remain fairly steady, but without regulation, it could change a lot with changing rpm or load.

These alternators are not designed to be used for extended periods without a functioning regulator. Continuous operation could lead to failure of the rotor brushes or the regulator circuitry that the rotor current passes through, and would mean loss of any output. That's why the Battery warning light is on when the sense fuse is removed.

Based on the FSM specs, at idle (~700 rpm), the alternator is designed to deliver at least 36 or 39 Amps (depending on transmission type). The alternator itself might not meet the original specs, perhaps due to age, or because it's an inferior replacement. Has it been tested for capacity at both low and high rpm and low and high loads to see if it meets the specs?

Similarly, if the battery is faulty, it could be loading the alternator excessively, leaving less spare alternator capacity to provide current for your added loads at idle. Has the battery been properly tested?
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