How do you define "holding a charge"?

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I have a couple of basic questions...

1.) Since all batteries will slowly discharge, how you determine if/when you're battery is "holding a charge"? I know that the ability of your car battery to hold or not hold a charge is one of the things you would consider before deciding to replace a battery, so I'm just wondering how you might make that determination?

2.) I see a lot of handy images for "battery voltage charts" - https://duckduckgo.com/?q=battery+voltage+chart&t=h_&iax=images&ia=images . *I* think there's a lot of variance on several of the charts between 100% charged, 50% charged, etc., etc. How do you know which one(s) are accurate?

Thank you,
Ed
 
Is "holding a charge" really a thing at all? Quite simply, a battery won't have 100AH worth of energy on day 1, and (as an example) only 30AH energy on day 5.

When we test batteries, we fully charge them, then test capacity. Lead-acid batteries tend to lose capacity both over time and through use.

For this discussion, a regularly used 44AH aviation battery might make as much as 53AH when brand new, 47AH at the 1 year point, 40AH at the 2 year point and 36AH by year 3. Being as it's still over 80% of nameplate capacity at year 3, it's still considered "good". By year 4, it won't pass the test and is replaced.
 
I used to go by the general rule that an automotive type flooded battery would loose around 1-2% of charge per day if there was no parasitic load on it. Most modern vehicles will pull a small amount of energy under normal circumstances and double that discharge. These are just general statements not specific to any particular vehicle and circumstance. The vehicles I used to work on would be dead in around 2 weeks if not used but they had radio equipment that used a similar "always on" draw to retain memory.
 
Is "holding a charge" really a thing at all? Quite simply, a battery won't have 100AH worth of energy on day 1, and (as an example) only 30AH energy on day 5.

When we test batteries, we fully charge them, then test capacity. Lead-acid batteries tend to lose capacity both over time and through use.

For this discussion, a regularly used 44AH aviation battery might make as much as 53AH when brand new, 47AH at the 1 year point, 40AH at the 2 year point and 36AH by year 3. Being as it's still over 80% of nameplate capacity at year 3, it's still considered "good". By year 4, it won't pass the test and is replaced.
That's helpful, thank you.

So, as a general rule, you replace batteries that won't measure 80% of the original capacity? Is that roughly accurate?

Ed
 
So, as a general rule, you replace batteries that won't measure 80% of the original capacity? Is that roughly accurate?
80% is the rule used for "design life". For example I have 2 sets of SLAs here. One is a 10 year and the other a 5 year. The theory being in a float charge scenario with light cycling they are designed to retain >80% of their capacity inside the design life.

My current car battery is ~46 months old and down to just over 50% of its rated capacity (when tested to 12.0V at a 20C discharge). Given my use case and environment I'll probably get another year or two out of it. When I replaced the last one after a "failure to proceed" while camping it tested at ~20% of capacity. It was ok around town, but couldn't sustain a 3 year old repeatedly waking the car up to "go for a dwive". This car draws ~20A when it wakes up, so it's hard on batteries.

The Aviation case is a different kettle of fish. You can't put a set of jump cables on at 6,000ft when the engine dies, so the 80% rule is hard and fast.
 
I would say that if I make sure that a battery is fully charged and it can't start the car in a day to a week, it's not holding a charge. That's not solely about the battery though, as it might include the effects of parasitic draw, which can vary by vehicle or even conditions. But I suppose many have different tolerances for what they will

But when I had a battery that I fully charged (using an external charger) and I couldn't start the next morning I knew it was toast. Or when I listened to the radio for less than an hour after a full charge, it wasn't necessarily about charge retention but inadequate charge.

However, I don't think it's solely about how much charge is in a battery, but about starting a car. That's the primary reason for having a car battery. There are different standards for battery capacity, and you could theoretically do several things like listen to the radio, use power locks, or turn on the map lights while it just can't generate enough current to start. So battery wear doesn't just affect total capacity, but also internal resistance.

 
My 2010 Jeep wrangler went 8 yrs on the original battery. The last year I kept a close watch on it but it started the Jeep every time even after sitting for a week or more. I knew it was getting weaker but it still functioned adequately.
 
i don't worry about batteries until the car cranks slow. seen plenty of cars start with 10v batteries (old cheap ones that lost a cell or just aged out) I've never had a battery die suddenly, there's usually signs for a while before you get stranded.
 
i don't worry about batteries until the car cranks slow. seen plenty of cars start with 10v batteries (old cheap ones that lost a cell or just aged out) I've never had a battery die suddenly, there's usually signs for a while before you get stranded.

Of course voltage doesn't start a car. It requires current. And in any case I don't think the open circuit voltage necessarily represents the voltage under load. I think it might be possible to put a hundred or so 9V primary batteries in parallel, and that would start a small car if it could provide enough current. Might cause a battery fire too.
 
My 18 month old battery failed last week with absolutely NO warning. Parked the vehicle, and when I went back to the vehicle the battery was completely dead. So it does happen.
 
Generally a battery is 12.6 V, when new and fully charged. I have a 6 year old battery that will charge to that but slowly drifts down to 12.2 volts in three days, but stays above 12 V as long as its above freezing. In freezing weather it will drift down to 11.8 volts but will still start the car. Of course a battery tester is the best way to access its condition. I’ll probably replace the battery when I give the car to my daughter this spring.
 
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Of course voltage doesn't start a car. It requires current. And in any case I don't think the open circuit voltage necessarily represents the voltage under load. I think it might be possible to put a hundred or so 9V primary batteries in parallel, and that would start a small car if it could provide enough current. Might cause a battery fire too.
i know, i usually check the voltage while cranking. and a bunch of 9v batteries would start a car, but have fun wiring them all. I've used 2 batteries that wouldn't start a car on their own and hooked them together to get the amps i needed.

you guys worry too much.
 
Generally a battery is 12.6 V, when new and fully charged. I have a 6 year old battery that will charge to that but slowly drifts down to 12.2 volts in three days, but stays above 12 V as long as its above freezing. In freezing weather it will drift down to 11.8 volts but will still start the car. Of course a battery tester is the best way to access its condition. I’ll probably replace the battery when I give the car to my daughter this spring.

I know we deal with rules of thumb for the open circuit voltage. I'm sure that my automated battery charger basically looks at voltage when determining the state of charge, but it might also check for current.

I've had batteries that I knew were gone. I could charge them to about 12.6V and the charger would go into maintain mode. Then when I pulled it and checked the voltage in an hour, it was less than 11V. If I waited a day, I couldn't start it without a jump or using the charger and immediately starting.
 
i know, i usually check the voltage while cranking. and a bunch of 9v batteries would start a car, but have fun wiring them all. I've used 2 batteries that wouldn't start a car on their own and hooked them together to get the amps i needed.

you guys worry too much.

That's basically what a jump starter box does. Most aren't adequate to start a car on their own. They're usually fairly small 12V sealed lead-acid batteries, although the newer types are li-ion.

They can be used in different ways. Just hooking a good battery in parallel to a bad one can charge the bad one to some degree. But normally they just provide supplemental current. Usually when I can't start a car on a dead battery, maybe the lights and power locks are weak, but still somewhat working. But then I try to start and I'll notice a weak crank, but more likely just a repeated clicking sound, which is supposedly the starter just turning on and off when it can't get enough current. But add that supplemental battery and it fires right up.

I do remember once I helped a coworker with a jump start with my car's battery. He was asking why I just connected it without starting the engine, and I said the battery alone should be enough and it started it. But sometimes I've had a somewhat worn battery, and I really needed to have my car running in order to jump start the other car.
 
I haven't seen it much with wet cell batteries, but with AGM batteries of the type used with UPS and alarm panels, and probably with vehicles too, when they go bad the usual failure mode is that they act like they have much less capacity than they should.

For example, if you put one of these batteries that appears to be dead and needing a charge onto a multi-stage battery charger, it'll go into float charge right away and consume very little current. I suspect that the cause is that the battery is sulphated and has a high internal resistance. If you then take it off the charger and measure it's voltage, it'll read 13V or more. But if you load test it, it won't even light up an 1194 bulb.

Probably in a vehicle a battery is usually condemned long before it gets that bad. The usual cause seems to be that the battery was left discharged for a long period of time.
 
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