Car Batteries

**enf** · 08-02-21, 01:34 PM

Do ordinary car batteries have a "deep charge"?

I charged up the battery in the spare car (old Barina) with the ubiquitous home charger, but a month of inaction has meant it's flat again.

There should be no problem as it's not that old.

I'll buy another battery if I have to, but I'd rather have this one charged properly.

Clues for the clueless?

**wotnot** · 08-02-21, 02:01 PM

Check there isn't any parasitic draw on the battery with car turned off

**Tom Mix** · 08-02-21, 05:05 PM

All lead/acid batts have self discharge. All we can do is keep em fit for their job which is mainly start, So if car not being used put charger on weekly/fortnightly to keep batt ready. Modern cars have constant load when off from CPU,which also runs em down. I remove neg batt term when car idle,
so charge remains. May need backup 12v to keep memory.

**wotnot** · 08-02-21, 06:16 PM

Originally Posted by Tom Mix

All lead/acid batts have self discharge. All we can do is keep em fit for their job which is mainly start, So if car not being used put charger on weekly/fortnightly to keep batt ready. Modern cars have constant load when off from CPU,which also runs em down. I remove neg batt term when car idle,
so charge remains. May need backup 12v to keep memory.

The self-discharge rate or your typical lead-acid car battery, is something like 10milliamphour or so (just sitting on a shelf unconnected)

Enf did say 'old Barina', but how that relates to vehicle systems standby current draw, I have no actual idea ; but I do know it'll relate to what trim level/spec of Barina it is, and a 'typical' current draw by a (modern) electronically equipped vehicle should be up to but not much more than 50milliamphour ~ one can math it out from there, based on the amp/hour capacity of the battery.

It's a tad misleading to 'constant load' when off (it's a very low standby current, and not really a load), and that 'CPU' is so responsible ~ invariably with modern auto electrics, it's an interconnected (networked) set of modules, and any one module can fail/play up, to cause excess current draw from battery, with the key off. One vid I saw online (PHAD), it was the wiper control module ; due to water ingress, the wiper arm park switch had failed, and also the wiper motor itself was seized -- with key off, every other module would go to sleep mode, except the wiper control module, which kept detecting the wiper arm wasn't parked, and dump power into the seized motor forever (hopelessly) trying the park the arm before shutting down...every few minutes..., even failure of airbag connectors, door-switches, BCM (body control module) can cause excessive 'off' state battery drain.

Which is why, especially with modern cars, it's best to check what the *actual* off state power draw of the vehicle electronics really is, or, is indeed a 'true' off state is being achieved at all. Some vehicles can take 2 or 3 minutes to reach shutdown/standby state, but if you see more than a continual 60 or more milliamps being drawn by the system after 5mins, chances are a module isn't turning off ; usually each module has it's own fuse, so you can play Russian-roulette with the fuse panel, until you pull out the fuse that results in a lower current draw at the battery ... then you might need a scanner to find out wtf that module is doing, but at least you've an idea where the fault may be.

Obviously, once you know the actual off state current being drawn, you can calculate whether or not the battery should be as depleted as it is, after so many weeks idle.

Note, most car batteries really don't appreciate being discharged lower than 10.5volts, and usually suffer damage during any high amperage recharge attempt from this low end of the battery spec (slow fill to 11.4v then bulk charge, car alternators just dump bulk current back into a depleted battery most of the time)

**VroomVroom** · 09-02-21, 07:34 AM

the battery may be sulphated , you can get chargers which can fix this to an extent , but the best way is to keep a 10w solar panel connected across the terminals to keep it topped up.

**Tom Mix** · 09-02-21, 05:39 PM

Yes wotnot, but I live in real world where theory matters not. Our goal is to get to destination & return. So actual conditions can vary from textbook-we just want to get to where we plan. Vehiclebatts as long as do functions will do till they won't. In that vein, batts do degrade in service but we can keep em running as long as we can, so what matters is actual self discharge at this time, & steps to get round it.

**enf** · 09-02-21, 05:53 PM

Thanks all. I've decided to charge it again for the time being.

Then I'll make sure I put on the trickle charger on a regular basis. Canberra has ALWAYS killed most batteries, as it's uber cold in winter and that is when the car was idle this year.

As I said, it's a spare car, but it's not much of ANYTHING if it won't start when needed....a 2001 Barina SB.

**Uncle Fester** · 10-02-21, 10:38 AM

Originally Posted by enf

I charged up the battery in the spare car (old Barina) with the ubiquitous home charger, but a month of inaction has meant it's flat again.

That battery is now likely sulphated and can not be recovered. It might still start right after a charge but not necessarily when you need it.

If your intention is to have a the car start immediately when it is needed(also in winter) then I would replace the battery now!
Then disconnect when the car is not needed for a while and perform a top up charge roughly once a month, although 2-3 months is still OK with a healthy disconnected battery.