Minimum voltage for starting?

[Peeeeda]

Truck was parked up for 3 weeks recently. Aux battery was sitting fine at 12.6v but the main battery was showing a steady 11.8v before I cranked it ( the main is 5 year old from new). The truck still turns over every day no problem. I'm just wondering how it can turn over with so little state of charge. I thought 12.2 volts = about 50% SOC. Every morning the batt monitor shows less than 12 volts. I know I'm going to have to fork out for a new cranker soon but just enquiring out of interest. Anyone (old Tony) know?

 

[Old.Tony]

The voltage is a reasonably good indicator of the battery's state of charge, especially after sitting idle for a few hours or more like yours was, but it doesn't tell you how many amps you'll be able to pull out of it.

Even a battery showing dead flat on volts (10.75) may still turn over the engine, it just won't do it as lively as a normally-charged battery (I can't say "fully" because car batteries are rarely fully charged).

The difficulty here is that by allowing the battery to drain that far, more rapid sulphation is encouraged and the battery life is shortened considerably. It is possible to revive these batteries using a C-Tek charger in "Recovery/Repair" mode but that's not going to last forever either. Sulphation is where the sulphur in the acid forms crystals which insulates the plates. Some break off and fall to the bottom and as they grow they eventually make contact with the plates of the battery and the battery surrenders. You can pick a sulphated battery quite easily - it won't accept a charge easily. You'll connect it to a charger expecting it to take (say) 8 hours to charge and it's all done in 2 hours.

You'll find that inverters will cut out at 11.75V. They're designed to do that so that they don't drain the main battery below the point where drawing more current is impossible - so you can get your car going again. The lower threshold of 10.75V is generally disastrous for the battery (in terms of the sulphation process described earlier) but the battery will still carry significant current-delivery capacity. It's not worth much for devices that need 12V, but your starter motor is a rather primitive device and as long as the ECU and the injectors can get enough juice to operate, even a technically dead battery could get you going again.

To avoid that, perhaps a slow trickle-charger (don't use a fast charger connected directly to the car's electrics) or one of those solar panels on the dashboard would help.

 

[Krankin]

I've had many batteries show 12 + volts but have absolutely no balls...
stick a meter across the terminals...it may show 12 v but then crank it (if it will) and watch what the meter shows then youll know if its cactus or not.

 

[Old.Tony]

A battery that is showing reasonable voltage but can deliver no current probably has a fair amount of sulphation on it. I should have included that in the bit about sulphation, although it's inferred where I talked about the reduced charging time - because the charger sees the higher voltage (indicating a charged battery) it stops charging, even though the battery's still fairly flat - it's just those parts of the plates that can still conduct electricity (non-sulphated areas) that are taking the charge, and with reduced area = reduced current.

 

[Krankin]

*** A tool that has been indespensible to me and I reckon everyone should have is a "Hydrometer"...checking the specific gravity of the electrolyte in each cell will quickly let you know the condition of each cell and the battery overall....only one cell down spells the end of whole batt....

of course for wet cell (lead acid)batts only.

 

[Old.Tony]

+1 on that, but the trend towards sealed batteries prevents it. The trend isn't a terrible one - AGM batteries can be charged exceedingly fast (C/3 charge rates aren't out of the question) which would gas a normal battery into oblivion, but the AGM battery encourages the recombination of hydrogen and oxygen, rather than its release. Extraordinary pressures will, of course, result in a release of gas, but that's not common. Some AGM batteries can be used as crankers (or 'SLI' - Starter, Lights & Ignition) as well. particularly the spiral wound ones.

Gels are sealed because there's no fluid full stop, so no need to ever open them. It uses a jelly (sulphuric acid and a silicate to form the gel) and that's its own downfall - bubbles form during charging right at the plate-gel interface separating the gel from the plate and effectively forming an insulator. For this reason, I think gels are junk and really need to be disregarded as a reliable long-term power source. You can't charge 'em fast, you can't discharge 'em fast, all you can do is trickle-charge them gently or watch them die.

But if you've got a flooded (old style) battery, absolutely use the hydrometer. I have one, but only have 2 flooded DC batteries left - and they're not long for the world, so my hydrometer will go in the bin in favour of the AGM technology I'm moving to.

 

[Krankin]

while there is and i reckon there will be for a while still the "old Style" i will be using them as apposed to the dearer other styles of "crankers".
as for batts needed for camping "deep cycle" etc, i have a supply of those from my work to suit my needs.
I personnaly wont be spending any extra on fancy batts for cranking, imo a waste of cash.

 

[Old.Tony]

Absolutely agree there - I won't replace my starter (which is a Nissan-supplied sealed one anyway) with an AGM. But it is nice to know that if absolutely necessary, I can remove the AGM from the rear, take it around the front, hook up jumper leads and crank the car over with the AGM with NO fear for the battery's welfare.

 

[Krankin]

Absolutely agree there - I won't replace my starter (which is a Nissan-supplied sealed one anyway) with an AGM. But it is nice to know that if absolutely necessary, I can remove the AGM from the rear, take it around the front, hook up jumper leads and crank the car over with the AGM with NO fear for the battery's welfare.

well theres reason 109 "for" a D22....room for a second batt in the engine bay....in the process of fitting a second cranker now after recieving my bits from Nizzbits...

will have to add that to the appropriate thread

 

[tweak'e]

its surprising how well starter motors go even with such low voltage.
however the problem you run into is the ecu low voltage cutoff. i think D22 is 11v and it will not fire the injectors despite the starter motor being able to crank the engine over fine.

 

[KraftyPg]

well theres reason 109 "for" a D22....room for a second batt in the engine bay....

Choose the right isolator and cabling and you can start the car without going to the trouble of removing the second battery from wherever it's mounted, so therefore being under the bonnet is only a bonus for those who actually want a second battery there.