All modern cars will have an amount of quiescent current draw to power things like the alarm, the clock, the radio presets and so on when the car is parked up and switched off. I've seen it written somewhere that around 50mA can be considered 'normal'.
My question is - if the quiescent current draw is 50mA (0.05A), how do I calculate voltage drop per hour?
For instance, if I park the car up at 10pm and the battery is showing
12.5V, with a 50mA draw overnight what will the voltage be at, say,
9am?
Part of the problem with batteries, is determining what is going on, and exactly how healthy the car battery is.
I have a lot of trouble with that.
For example, I've had many batteries, where I get out in the drive, hook up the dumb charger, and after two minutes the stupid thing measures 18V. And you just know that's wrong, the battery is not "stiff enough", it's only got a fraction of the normal ampere-hour (Ah) capacity. Yet, for maybe several weeks, it continues to turn over the car. But you know its days are numbered. And how do you tell one 18V charging session from another 18V charging session (where it's failed and won't start the car) ?
There are a ton of articles to entertain you.
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OK, so the second article says the end-of-discharge voltage is 1.75V. Six times that is 10.5V .
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A cell voltage of 2.10V at room temperature reveals a charge of about 90 percent.
Six times that is 12.6V and that's after you've disconnected the charger and let it stabilize for 12+ hours or so. Right after it is charged, it reads higher than that, and taking an open circuit voltage reading is then "not representative" of battery state. Your smart charger does its best to terminate charge at the right condition ("topping"), but the actual working voltage is not visible until the electrolyte has settled and equilibrated (sulphuric acid the same composition, right at the plate surface, as 5mm away from it). Using Specific Gravity (SG) with a liquid sampling device, is about as useful as a voltage reading, and even SG can be deceiving if only half of the plate (down deep) is working.
For practical discharge ("running a golf cart"), you limit the depth of discharge to about 25% on lead acid. The 10.5V value, would be if you were designing a "cutoff circuit", to disconnect the battery from the load and stop all additional discharge.
25% depth of discharge is preferred, to maximize the number of charge cycles you can get from it.
Lead Acid batteries are inefficient at high load. This is captured in some of the discharge curves shown for UPS (uninterruptable power supplies, sealed lead acid SLA). If you run a UPS at the rated load, maybe the battery lasts for 2 minutes. But the relationship is non-linear, and if drawing 50mA, then you get the ampere-hour rating of the battery back.
50Ah is 50000 mAh, divided by 50mA is 1000 hours. Whereas if you drew 50000mA, you might be expecting
1 hour (60 minutes), but the stupid lead acid battery only lasts for 20 minutes.
Summary: At the low load you describe, if the battery is brand new, mint condition, fully charged, it might last
1000 hours before it hits 10.5V (and can't crank the car).
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To crank a car, some percentage of charge is needed, because the battery impedance matters and it gets "weak in the knees" at low charge. For example, I did a test using two metering devices here. A clamp-on DC ammeter with peak hold. A voltmeter with peak hold. And when the car starts, the values I got were 9V @ 150A . The battery voltage then, was 12.6 internally, but Ohms law saw to it that 3.6V dropped across the "resistor" inside the battery. 3.6V/150A is 24 milliohms. When drawing
50mA, there's practically no voltage drop at all, from the ideal internal value.
I would shoot for 25% discharge, so if the battery is
60Ah, that gives you 15Ah to work with, 50mA load is
300 hours or 12.5 days. And then, perhaps the car will still start.
There are some examples here of "pulse-challenging" a battery. And starting a car is a rather large pulse. Car batteries are designed to provide CCA, they're not a stationary storage system, like keeping your house warm for 10 days. They're actually intended for short term use, cranking the starter. Whereas a golf cart battery is "motive storage" and could consist of many batteries and many amp-hours. And it might take all day, to make a dent in the bank of batteries.
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Getting back to the voltage, here are some values.
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SG
100% 1.265 12.65V <=== 26°C (78°F) after a 24h rest
75% 1.225 12.45V Voltage is a strong function of temperature,
50% 1.190 12.24V and temperature compensation is required
25% 1.155 12.06V in the real world, like when it is -20C in the drive.
0% 1.120 11.89V
10.5V is cutoff (hard on the battery, could reverse bias something).
10.5V is not a useful value for our home maintenance plan.
We're more interested in voltages that will start a car.
11.9 or 12.0V is about as low as you should go, before turning the key and starting. The battery needs some juice in it, to be useful. You can run a 20mA LED all the way down to 10.5V because the battery impedance doesn't matter for ridiculously low loads. We need that "stiff 24 milliohms" value, when pumping 2 horsepower into the starter motor.
When I had a car battery fully charged, and a suspected starter problem (on a four cylinder car), the measurements during starting were 9V @ 150A. A four cylinder starter should probably have been drawing 100A, in which case the battery voltage would not have dropped so low. That's at the point where the starter is heating significantly. And you'd give it a lot of time between "trials" :-) Cars which start easily on the fourth crank, might not be so bad.
For max life then, I could run the battery, day after day, from 100% full to 75% full. This would (perhaps) maximize the number of charge cycles. Like, if I was running a golf cart, that would be a useful operating range.
If I run the battery from 100% ro 25%, that's 75% charge difference, if the battery is 50Ah times 0.75, I have 37.5Ah or 37500mAh, divided by 50mA, 750 hours. Round and call it a month. That means, if I do that for a month, the car will barely start, the starter will be getting a bit warmer than normal.
But then, real batteries (and you probably have some idea how poorly the current one is doing), If I was connecting my charger and seeing 18V while it charged, I would be out of my fricken mind to be waiting a month to test. I'd be lucky to get three days our of it at 50mA. When they're on their last legs, the vampire load really hurts them.
Ya see, it's like predicting when a relative is going to die :-) You're hoping hoping... and before you know it, they're 92 :-) You can sorta tell from some of the symptoms, that the battery "is not well", but it's damn hard to tell "only three days left" or "won't be able to crank the car a second time". That's why we don't try to run them right down to the wire.
You can see in my worked results, I'm mostly interested in the ampere-hours and what percentage of the battery I've kicked the snot out of. While the voltage gives some indication of the remaining charge, you must allow the battery to stabilize before reading it. Whether charging or discharging, that disturbs the acid and plate surface conditions.
The 11.9V or 12.0V minimum starting voltages, that is at 25C, and temperature corrections must be applied at temperatures other than the "room temperature". If you don't apply temperature correction, you can be off by 50% of battery capacity! (Mistake a reading of 100% full, for actual
50% full.) Good smart chargers have a thermistor, and measure temperature and attempt to do at least a little bit of correction when charging. Equipment at 5X to 10X the price, comes with a thermistor on a cable, to be clamped to one of the electrodes in an attempt to measure temperature.
Even hydrometers, for measuring SG, you need to run your finger across the chart, for the right temperature.
Just what a battery reads voltage wise (to decimal points) has a lot to do with the individual battery. You can work out how long the battery will last by its capacity in amp.hrs. And knowing the quiescent load. Using that, discharging by half seems to be the maximum that will allow the car to start OK. As a very rough guide.
But the capacity of a battery deteriorates with age.
All true, and it also depends on how the voltage is measured, and the type of battery.
My car has automatic engine stop when the footbrake is on and the car is stationary. My understanding is that the battery is specifically designed to cope with repeated starts. There were a number of conditions where that would not apply (such as A/C running), but also where the battery capacity and/or voltage was not deemed sufficient (by software?) to restart the engine when the brake was released.
Over the last year the "battery insufficient" symbol has been appearing more regularly, and now the automatic engine stop will not work at all. The car (and battery) is 5.5 years old, so I guess it's battery age which is preventing the autostop working. The car always starts perfectly first time, even when it hasn't been used for a week or so. It would be interesting to see what parameters are required to be met for the "battery insufficient" symbol to not appear.
During the first lockdown my car had little use although on fortnightly trips to the supermarket I took it for a long drive around the block to get it up to temperature and to exercise the A/C. During this time the stop/start function was always (automatically) disabled. It took 100+ mile non-stop journey before it started working again.
You cannot do it like that. The voltage curve on a battery drops very little for ages, then if any of the cells are lower capacity than others there is a quite sudden drop by 2v or so, and at that time, the low cell is being reverse charged by the current passing through the load. Not a good thing to occur.
What somebody electronically minded did on a battery where you can actually get at the cell interconnects, few these days, sadly, he put a monitor so it looked at every cell on its own and then he could tell the weak cell or cells, but as to what to do with such info, who knows? I'm surprised modern car electronics do not already allow this like they tend to do on Lithium cells these days. Brian
Good article that, Fred. I knew by experience taking a spot reading of lead acid voltage of little use. But didn't know the makers reckoned it had to be left unused for 24 hours.
Given the cost, makes sense to have it tested first? There are sophisticated testers that give an instant readout of the condition. A bit too expensive for DIY, but a decent spares place should have one. ACT is one such.
However, if you charge the battery with one of those £14 Lidl chargers, and it struggles to start the car, there's a very good chance it is faulty.
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