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Jack the R
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PostSubject: Re: Thermistor    Tue Sep 27, 2016 9:49 pm

albertj wrote:

Battery temp not necessarily same as interior car temp due to heating while charging; have to have some way to detect an prevent thermal runaway if you're going to boost the charge rate.  

That's a good point. I've never used a temperature sensor when using a battery charger though. Is the alternator charging at a higher rate than a battery charger?
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albertj
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PostSubject: Re: Thermistor    Wed Sep 28, 2016 1:08 am

Jack the R wrote:


albertj wrote:

Battery temp not necessarily same as interior car temp due to heating while charging; have to have some way to detect an prevent thermal runaway if you're going to boost the charge rate.  



That's a good point.  I've never used a temperature sensor when using a battery charger though.  Is the alternator charging at a higher rate than a battery charger?


I don't know, and I don't think it matters because there are other ways to figure out how to goose the charge rate.  Here are some comments that may be of interest:

1) BTW I just bought a new battery charger, a Stanley.  You can see it as Walmart #: 552936580 on the walmart web site.  Surprise, surprise... it works well; it has a reconditioning cycle that uses frequency pulses to break up sulfation and extend battery life; it automatically modulates charge rate - apparently as a function of the state of the charge on the battery as it is charging (it is possible to do that).  My guess is that as the battery charges and the voltage at the terminals goes up, this charger trims the charging amperage downward.  This could be done by switching off the charge and switching in a circuit to measure battery state, using that information to adjust charge rate.  I imagine it done with a microprocessor (think BASIC STAMP not PC) and some switching circuitry.  No idea how to adequately protect that circuit from reverse polarity at that battery charger's price point, though, so I'd be diligent about polarity. Very diligent. Making some level of current available to the battery, for charging, isn't that hard.  

2) An alternator can economically charge a battery at a  MUCH higher rate than a household or small commercial battery charger.  At idle even, you can get 50+ amps out of an alternator depending on how it is set up.  At speed, you can get the rated amperage (80, 100, 140, or what have you).  Suppose you are getting 80 amps at 14 v +/- from the alternator.  You could charge a car battery well within a couple hours if deeply discharged; in minutes (I am thinking 15-20) if it was well-charged but used to start the vehicle (starting is a fairly big hit on a battery).  Using the thermistor arrangement you could easily maximize the charge rate with essentially no guesswork and no switching circuitry to add electrical noise to the car's systems -- important due to the PCM, BCM and other electronics.  

3) Why the thermistor?  Because in practice car charging systems are usually designed to top-off good batteries not deep charge them, and the Riv has a high parasitic drain.  Meaning that in a normal drive cycle it probably needs a pretty good recharge for the battery not just a top-off -- in order to make up not only for the start but also for all the power that got used when the car was just sitting "off." (hint - the Riv is NEVER really all the way Off...)    Also by the way charging the battery so hard that it overheats causes big problems,  they will emit lots of hydrogen gas while they overheat, and if they survive the fast cycling it won't be for long.  Basically the "average" onboard auto electrics are sized to run the accessories on the car and as well reasonably quietly charge the battery. The Riv is above average in that respect and will more quickly charge its battery by boosting the charge rate somewhat, modulated by that thermistor.  

Albertj
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