Author Topic: Is there a way to safely charge a car battery with this equipment? If not, why?  (Read 1711 times)

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Offline Yansi

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Guess what: 27V just tells you those alternators are probably shot, as with 27V (13.5V) you are NOT going to charge the battery, especially during the typical couple of hours of motor run time. Understood?  :-//
 

Offline Simon

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Online T3sl4co1l

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Ah, well there you have it, I left the lights on last night :-DD so I've got it indoors charging right now. Taking a good 12.3A at 13.63V right now, slowly rising (max 14.20).  Was about 12V OC, should be here most of the afternoon then I'll put it back good as new.

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Offline Simon

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shhhh don't tell Yansi you can charge with less than 14V, you will be on his naughty list too...
 

Online T3sl4co1l

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Well, so what? It's taking current at a lower voltage because it's in a discharged state.  As the concentration of Pb-->PbO2 and PbSO4-->Pb rises, the cell potential also rises.  (In fact it's up to 13.74V already.)  When all surfaces are fully charged, the voltage levels off somewhere around there (when under some degree of charge, because again, overpotential).  I'm going to float it at 14.2 for a while (preferably 12-24 hr, not sure if I'll get that much time on it tonight) to ensure a full charge (desulfation, to the extent that it can be performed at all).

Tim
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Offline Simon

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Yea, and all a battery does these days is start an engine which takes less than 50Wh and is quikly replenished.
 

Offline Nusa

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Yea, and all a battery does these days is start an engine which takes less than 50Wh and is quikly replenished.

Under ideal conditions, perhaps. It also has to be able to start a cold engine in adverse conditions. Try it at -18C. 60% of the battery capacity is gone because it's cold. The oil in the engine is a cold thick fluid, so every moving part takes way more energy to move. In fact, that's so important that those are the conditions car batteries are rated under. The CCA (cold cranking amps) is how much current a very cold charged battery should be able to deliver for up to 30 seconds. When it's new.

Now try it at -40C. Yes, I've really had to start vehicles at those temperatures. Better to stay inside if it's not important, to tell the truth. People who deal with such climates regularly have engine and battery heaters. And replace batteries much sooner than those in warm climates.
« Last Edit: October 20, 2019, 11:29:14 pm by Nusa »
 
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Online T3sl4co1l

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Heh, speaking of low car batteries and even lower temperatures -- last year we got some -20 days, one of which I had to go to work on.  Well, car wouldn't start the night before anyway, so I took the battery inside and floated it.  Next morning put it back in, I guarantee you my room temperature, fully charged battery gave me the easiest start of anyone in the whole city that morning.  It didn't seem to bog down from the cold oil at all. ;D

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Offline Simon

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Yea, and all a battery does these days is start an engine which takes less than 50Wh and is quikly replenished.

Under ideal conditions, perhaps. It also has to be able to start a cold engine in adverse conditions. Try it at -18C. 60% of the battery capacity is gone because it's cold. The oil in the engine is a cold thick fluid, so every moving part takes way more energy to move. In fact, that's so important that those are the conditions car batteries are rated under. The CCA (cold cranking amps) is how much current a very cold charged battery should be able to deliver for up to 30 seconds. When it's new.

Now try it at -40C. Yes, I've really had to start vehicles at those temperatures. Better to stay inside if it's not important, to tell the truth. People who deal with such climates regularly have engine and battery heaters. And replace batteries much sooner than those in warm climates.

In which case as Tim mentioned above the battery still charges at less than 14V. I think if a military vehicle can use 27-27.6V (13.5V-13.8V) despite having fuel fired battery and engine heaters for when it's really cold  then that's about good enough.
 

Online tautech

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Simon, you overlook a couple of things, automotive charging systems have been regulated to 13.8 - 14.2 since forever for FLA 12V batteries and the max voltage any charging system can deliver is also current regulated.

These 2 things control what a charge system can deliver and it's also governed by the SOC of the battery.

All that has changed in recent years is the battery chemistry and the max is more likely 14.4 - 14.5 V these days.
Still, any checks on auto charge systems need be done with a battery with a high SOC so that the charge system is NOT current limiting and then the charge cutoff can be accurately measured.
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Offline Simon

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If it's an actual battery management system like some modern cars have then fair enough, it takes care of itself like restarting the engine if you are sitting in traffic running you electric air conditioning and the battery gets low. But run-of-the-mill vehicles that just connect everything in parallel don't run that high. My last car ran 13.8V I had a constant display on it.
 

Online tautech

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If it's an actual battery management system like some modern cars have then fair enough, it takes care of itself like restarting the engine if you are sitting in traffic running you electric air conditioning and the battery gets low. But run-of-the-mill vehicles that just connect everything in parallel don't run that high. My last car ran 13.8V I had a constant display on it.
Understood, however I'd never trust a dashboard meter to be particularly accurate let alone the average vehicle owner having the knowledge to properly interpret it. Charge failures need prompt alert to the driver and I wouldn't care for anything other than a red charge light alert.

Still, that 13.8 V display is typically the lowest max you want to see a FLA charging system maintain.
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Online Mechatrommer

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older battery having trouble reaching 13.8V let alone 14.4V. i monitored few battery and with 14.4V charger, some of them can constantly suck 0.25-0.5A and get hot. hot battery is not a good sign (hydrogen discharge) i boomed one of those 60AH'er right next to my ear while welding its rusty holder to a car with it sat right next, and being (over?) charged (13.8 - 14.4V) overnight. i got a warning of pissing smell, and read about hydrogen discharge a night before, and remember did a hydrogen pop experiment in school decades ago. a real stupid move i didnt take the hints, now i know and the puzzles now all connected together nicely and it will stick in my head pretty well i guess. sometime i believe a little bit of knock on our head or ringing bell echo from the ear can improve memory retention and attention, even on aging brain.
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Offline Simon

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If it's an actual battery management system like some modern cars have then fair enough, it takes care of itself like restarting the engine if you are sitting in traffic running you electric air conditioning and the battery gets low. But run-of-the-mill vehicles that just connect everything in parallel don't run that high. My last car ran 13.8V I had a constant display on it.
Understood, however I'd never trust a dashboard meter to be particularly accurate let alone the average vehicle owner having the knowledge to properly interpret it. Charge failures need prompt alert to the driver and I wouldn't care for anything other than a red charge light alert.

Still, that 13.8 V display is typically the lowest max you want to see a FLA charging system maintain.

It was a CAN bus display so it would have been either taking it from CAN bus data or i would hope that someone that can design a CAN bus device would be clever enough to measure voltages to 0.1V!
 

Online T3sl4co1l

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Simon, about an hour later and battery voltage rose to the set 14.25V (as it turns out) and current started to decline.  After about three more hours, it was still taking 1.5A charge, where I decided to cut it.

Doing the full charge at 13.8V will certainly (for the wet lead acid formulation) result in an incomplete charge state, and getting there will take exponentially longer than doing it at a slightly higher voltage.

Tim
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Offline Simon

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well I expect cars err on the side of caution. You can't drive around indefinitely at 14.4V as you will knacker the battery. Given that the battery just works to start the car in most use cases 13.8V float is probably deemed to be a compromise. If i am manually charging i too will use in excess of 14V for speed with a current limit but unless the vehicle has a BMS they have to compromise. The alternator current limit is to protect the alternator not the battery. Modern Audi's have a load of BMS gear attached to the battery i am told and it looks after the battery to the point of restarting the engine if it's off and the ignition is on and the battery voltage is dropping.
 

Online tautech

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Well, so what? It's taking current at a lower voltage because it's in a discharged state.  As the concentration of Pb-->PbO2 and PbSO4-->Pb rises, the cell potential also rises.  (In fact it's up to 13.74V already.)  When all surfaces are fully charged, the voltage levels off somewhere around there (when under some degree of charge, because again, overpotential).  I'm going to float it at 14.2 for a while (preferably 12-24 hr, not sure if I'll get that much time on it tonight) to ensure a full charge (desulfation, to the extent that it can be performed at all).

Tim
You don't get a maintenance charge until 15V (gassing charge) where you'd get any benefit from desulphation and really you don't want to go there with the modern sealed FLA.

IMO the old noisy generators and their spikes where better at battery desulphation than the modern alternator.
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Offline Simon

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I thought the old "generators" or dynamos were supposed to be smooth DC. It's all you hear about from the old cranks that want to keep their dynamos that they are better because of the smooth output. In reality the 3 phase that get rectified are pretty good
 

Offline soldar

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Old (dynamo) regulators were extremely simple. Two relays. One would close above a certain voltage and would disconnect the generator below that. This was to prevent the battery from discharging through the generator. A diode could do the same job today.

Another relay kicked in above a certain charging current and opened the NC contacts which put a resistor in series and this limited charging current.

That's it.

You could have all sorts of problems if contacts got stuck or voltage or current activation got out of whack.
« Last Edit: October 21, 2019, 03:48:57 pm by soldar »
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Offline Simon

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yea, but all the old farts still harp on about the good old days of dynamo's
 

Offline floobydust

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Any suggestions for equalize charge voltage on FLA? Trojan gives 2.7Vpc for 16.2V which seems pretty high.
I had 12.7V at rest and levelled off at 14.7V at 0.5A (2.45Vpc) overnight.

One particular car maker has ~135mA parasitic drain due to the ECU. The batteries would barely last 3 years. I think a firmware fix got it down to 70mA. An ECU typically does a fuel-pump run to cool the fuel rail (anti-percolation), a canister-purge after parked for hours, TPMS checks etc. - lots of stuff after key off which drains any surface charge.
 

Offline Simon

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Well my car tonight according to a cheapo USB charger went as high as 14.5V dropping to 14.3-14.4. However I did leave the dash cam on all weekend and do short journey's so assuming the alternator can be controlled maybe it knows that the battery is low (started at 12.3V). I'll see how it performs over the next week
 

Offline Yansi

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What car maker does have 100mA drain?  Such ECU would not pass even the basic LV124 tests.  ;D
 

Offline Yansi

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Well my car tonight according to a cheapo USB charger went as high as 14.5V dropping to 14.3-14.4. However I did leave the dash cam on all weekend and do short journey's so assuming the alternator can be controlled maybe it knows that the battery is low (started at 12.3V). I'll see how it performs over the next week

So the car charges at over 14 volts after all?  :-DD 
 

Offline Simon

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Like i said the battery was low. And like I said this is not universal, my last car was 13.8V and commercial vehicles 27-27.5. Lets see how it goes over the week as the battery charges back up again from the weekend abuse.
 


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