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Lead acid start battery tech and ESR meter to test auto battery cables

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fastline

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Long story short, my dad has been fighting a battery issue in his truck for a while. Sits for a week and then hardly enough to roll it over. I checked the battery voltage from inside the cab and with just the head lights on, it drops to 11V, during start, it drops to 9.3V. I am thinking junk battery since I just charged it.

I brought the bat home for a bench test and with 10A of load, the bat drops to 12.5V!! That pretty much clued me in to bad cables or terminations. So, how about using my new ESR meter to test the resistance in the cables?

Also, I really need to get some firm numbers regarding lead acid start batteries in terms of charge rates, times, etc. I know they are different than deep cycle because they just dump all their charge at once so the CCA is not really a good estimation of capacity to a degree. IE, if a bat has 1000CCA, what kind of load/Vdrop should I be looking for?

Since I do some wrenching, I know under starting conditions, a bat dropping below 10.5V is something to be concerned about but especially in this case, there is little more to it. I do know the start current in this app was 360A.
 

adamey

Member
Simplest way is to do a voltage drop test. This site gives a really good explanation that's easy to follow.

Starter Voltage Drop

They say 0.2V is the maximum allowed, but on some cars with long cables it could be a little higher (like 0.3V). It shouldn't be over 0.3V, though.

However, I'd do a drop test between the battery post and the terminal first, as this is a very common problem area (and also easier to check). Drop from post to terminal would be the lowest reading since there's no long cable to increase resistance.

If you have those terminals where the battery cable gets "crushed" in between the terminal and a plate, then I'd look here as well, as these are very bad for corroding internally. I've cut a couple inches off to redo these things countless times over the years.
 
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KeepItSimpleStupid

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Most Helpful Member
I agree with the post above. I do it all the time.

Are you saying you have a phantom drain on the battery?

There is always some from the clock, ECM and radio presets. 10 mA is probably too big.
 

fastline

Member
Actual draw to the bat is 48mA which I thuught was in line but maybe not... THoughts? We tried uplugging about everything and could not kill that draw.. ECM, alt, pulled every fuse in the cabin with a no-go.. Maybe that is the issue, gotta find that draw. I figure 50mA would be fine..
 

KeepItSimpleStupid

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Thoughts are that 48 mA is too high. Think of it this way, your looking for a 250 ohm load. 10 mA would be a 1200 ohm load.

Unfortunately, it could be nearly anything. The alternator even. Starter. If you have access to fusible links that are removeable, I'd start there plus I'd take the alternator and starter out of the circuit.
 

adamey

Member
You need the wiring diagram to the vehicle. There could be any number of circuits that get power straight from the battery and have an inline fuse not part of the fusebox. You can also have fusible links or a secondary fusebox somewhere. Without a diagram you could be looking forever.

I'd recommend Alldata or Mitchell On Demand (Mitchell is better for wiring). You should be able to get online access to your vehicle for 3 days for $10 or so (print all the diagrams to keep).
 

MikeMl

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I already told you that the parasitic drain is way too high in your other thread
 
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tcmtech

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I wouldn't call 48 ma too high myself.

On a good 50 amp hour battery that still gives a 1000+ running hours which is around 42 days.
I have seen very few new cars with small battery's that can sit for much longer than about 6 - 8 weeks without needing a jump start.

IF the battery is dead after a few days with a 48 ma load the battery is bad.

Relating to cable checking I would do a voltage sets while cranking right across the battery and then one right across the starter. If the battery is under 11.5 volts while cranking its week and if there is more than a .5 volt difference between battery cranking voltage and the starter cranking voltage your cables are either bad or undersized or both.
 
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