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Building on the intelligent charger thread - I have an old ctek, that seems to work well, but I have a few batteries that it cycles and charges - but which I’m assuming are too far gone. They don’t hold charge for long and if I put anything that needs to draw a lot of amps on them - they fall short.

So - it struck me it would be useful to have a version of those things they AA carry for checking battery health ... cheaply if possible .... Has anyone experience or recommendations?  Even a cursory look on Amazon appears to show a bewildering array of devices.

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A load tester? If they're what you're thinking of I believe they just throw a large load across the battery and measure how many volts it manages to hold up.

A standard multimeter and a headlight bulb will do about as well as anything else. There's cheap digital load testers all over eBay although some are less safe than others.

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Can't get much easier than this:

At the workshop we have a device that has a big resistive load in it, and a gauge showing the voltage while the load is applied. Works pretty well too.

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I've heard good things among the Jag owner types that have a car stashed away for special occasions that the CTEK Car Battery Comfort Indicators are pretty good at £10.

https://www.halfords.com/motoring/battery-maintenance/battery-accessories/ctek-car-battery-comfort-indicator-166747.html

There's one that works through the cigarette lighter I wasn't aware of, too : https://www.halfords.com/motoring/battery-maintenance/battery-accessories/ctek-comfort-indicator-cigarette-plug-549784.html

Harry Metcalfe showed his considerable fleet in one video were all using them so he could see charge state at a glance.

I have a battery in mine that's nearly unable to hold charge so thinking two of these between the 90 and Jag and one CTEK charger is good value.

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They're neat enough but I suspect doing nothing you can't do for £1.50 in a more informative manner:

https://www.ebay.co.uk/itm/122810073088

There's panel / flush / round variants as well as LCD ones with a battery symbol etc... there's also cig lighter variants in all the colours which can look quite neat.

s-l1600.jpg.995e6f32702b106faccd575fe1109303.jpg

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I bought one of the USB port plus voltage display devices as shown above. However I have resisted fitting it so far, as I prefer my charger ports to be permanent live rather than ignition switched. I appreciate that this display must use very little power, but it is nonetheless a drain, which on an infrequently used vehicle is a bad thing. Obviously the ideal solution would be to run it through an independent switch, which is something else I need to find a round tuit for 😅

As an aside I also found the voltage displayed is not the same as my expensive multimeter.

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12 minutes ago, mickeyw said:

As an aside I also found the voltage displayed is not the same as my expensive multimeter.

Not surprising - but in use it doesn't matter if it's a little off as all that really matters is that you can see when your battery is low, and that your alternator brings the voltage up to a healthy level.

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There are 2 aspects to battery-health; the ability to store charge and the ability to deliver that charge rapidly-enough to actually start a vehicle.

Testing these - particularly with modern batteries - is rather more than just looking at the battery's terminal-voltage; I'm sure you're all familiar with the battery that's got enough voltage to bring the instrument-lights on at a seemingly-OK brightness but which gives the solenoid-death-rattle when you try and crank the engine.

My "general purpose" battery-test is to turn the headlights on - and leave them on for an hour. Headlamps typically take 5 Amps each - so that's 10 amps, plus maybe another 5 Amps for sidelights/number-plate-lights/instrument-panel illumination.

An hour of this is 15AH of load; a LR battery is typically rated for 85 or 110AH - so draining 15AH is less than 20% of the battery's rated capacity. If it can't start the engine after that, well, I'd be replacing it.

The kind of "big resistor" load-testers the AA use are OK to indicate the battery's ability to supply lots of current for a short period, but not to show how the battery will cope with extended discharge.

One thing to be careful of - decades back there were some such load-testers that had a central rectangular case containing the load-resistor and a good/bad-indicating meter, with a sort-of pair of tongs with probes you were meant to stab across the battery-terminals. Problem was, this invariably created a spark - not something you want close to a battery where there may be explosive Hydrogen gas around. A relative blew-up a tractor-battery using one such tester. Proper "load testers" these days have big alligator-clips and cables like jump-leads, the resistor/meter box with the test-switch can then be placed a few feet away from the Hydrogen-bomb before you start drawing big currents.

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@Tanuki has it right, you can't tell much about a battery with a single "simple" test any more than you can tell the quality of a cake by taking its temperature.

This graph shows the battery voltage on the same battery in "good" (but used) condition and in "dead" (needs replacing) condition with repeated loads being applied over a period of hours.

batt_graph_trojan31agm_good_vs_bad.png

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Interesting graph. It also shows that the simple, cheap load testers doe offer some info: if the voltage drops too low the first time the load is applied, the battery has had it. If voltage only drops off after several cycles/hours, it's harder too test and would need something like the testrig that produced the graph. I'm quite happy with a simple tester as a quick indication of condition.

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10 minutes ago, Escape said:

Interesting graph. It also shows that the simple, cheap load testers doe offer some info: if the voltage drops too low the first time the load is applied, the battery has had it. If voltage only drops off after several cycles/hours, it's harder too test and would need something like the testrig that produced the graph. I'm quite happy with a simple tester as a quick indication of condition.

The wrinkle is that "too low" is a factor of the battery capacity & the load applied - if you apply a load to a smaller perfectly healthy battery it may well drop as low as the graph for the "bad" battery. Also, some batteries can still produce significant cranking current but be at less than 25% of their original Ah capacity.

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Super helpful Fridge.
 

Is there another measure here - that knackered batteries seem to lose their ability to draw significant amps when charging from a discharged state?

I’ve notice both inability to handle load and inability to recover normally from being discharged in my own knackered batteries.

I think when I opened this thread with a question about a battery tester - I imagined something that could measure both in some way that was proportional to the battery size and capacity ..

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23 minutes ago, Anderzander said:

Super helpful Fridge.
Is there another measure here - that knackered batteries seem to lose their ability to draw significant amps when charging from a discharged state?

Very hard to say - @TSD had a totally knackered optima that still cranked his TGV lump over perfectly... but only once. If you tried a 2nd time it was dead flat, and I think he put it on the test rig and measured about 20-30Ah capacity on a ~100Ah battery. It believe would still accept lots of amps for charging too

I'd guess/imagine other constructions of battery, especially leisure ones, would sulphate up and get gradually higher internal resistance but it's a guess. Batteries are complex buckets of chemicals so it's hard to tell what's really happening by measuring volts.

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That Optima was measuring something like 16Ah before I even noticed a problem. I think that was what led to me writing that battery testbench code in the first place. Copying some published charging regimes for intelligent battery chargers, the testbench recovered the battery back to something like 35Ah. Still dismal for a 50Ah battery, but twice the initial state. I don't recall what the charging current was.

I have seen AGM batteries that struggled to accept any charge - I recall having to put about 30V on one Optima to get it to accept 33A (the current limit of the power supply I was using). That's plainly abusive, but then the battery was obviously in trouble anyway, and I just wanted to start the engine and go home :lol:

There are some systems which measure the battery impedance (like the internal resistance but more complex :ph34r:) to gauge battery health, but like you say, batteries are complicated and there probably isn't really one instant measure to be made that would give a definitive yes/no answer.

The best information might be to measure long term changes in the battery on the vehicle, and I think your battery gauge prototype could do it, but the software could be quite complex to implement. Terminal voltage, current and temperature should make it possible to track changes in internal resistance, charge acceptance, resting voltage and charged voltage in the long term. Hard to put a usefull pass/fail on any of those, but knowing how much any of those has changed since the battery was first installed could be a good guide.

But in the real world, I'm not sure it adds much over a half decent voltmeter directly on the battery terminals. The problem is still 'How much is too much'. If the expensive gauge says the battery resistance has risen 25% since install, but the car still starts, how many people would change the battery, and how many would just 'keep an eye on it'?

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3 hours ago, TSD said:

The best information might be to measure long term changes in the battery on the vehicle, and I think your battery gauge prototype could do it, but the software could be quite complex to implement. Terminal voltage, current and temperature should make it possible to track changes in internal resistance, charge acceptance, resting voltage and charged voltage in the long term. Hard to put a usefull pass/fail on any of those, but knowing how much any of those has changed since the battery was first installed could be a good guide.

Could maybe be as simple to see what goes in vs what comes out? As I've understood from this thread a dead battery happily accepts many amps, but doesn't give them back?

So once you get 10Ah out and 20Ah in, you know something is off.

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When I was writing my last post, I deleted a whole paragraph on 'coulomb counters'  as that's a whole new world of pain, even though the idea is attractive. The problem I see is that accumulated errors quickly make a mockery of the measurements, because of the range of measured values.

Very back of the envelope guesswork... 10 seconds of cranking at 500A is about 1.4Ah. Over 24 hours, that's about 60mA. So to have a hope of 5% accuracy, even over 1 day of use, needs a measurement accuracy of about +/-3mA, which wouldn't be easy for a sensor that doesn't saturate below 500A.

There's loads of 'yes, but...' on both sides of the problem, but it's probably a lot more difficult than it first appears. FridgeFreezer and I have discussed it at length many times, and I don't think we ever came to a scheme that we thought would really work reliably.

OTOH  It could work reasonably for a scheme like the 'house' battery in Fridges ambulance. The ratio between charge and discharge currents is much lower, and you know you're only trading depth of discharge against long term battery health - it's not super critical, but you know that 75% charge is no problem, but if you discharge below 50% you'll probably have some effect on the long term battery survival. So when your gauge says 60%, you might consider tolerating a slightly warmer G&T for the rest of the evening :lol:

I did try fitting a commercial (Xantrex) meter that measured charge in/out on some equipment I built years ago, but I don't really know how successful they were, never got any feedback from the end users.

 

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29 minutes ago, TSD said:

So when your gauge says 60%, you might consider tolerating a slightly warmer G&T for the rest of the evening :lol:

NEVER!!!

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  • 2 weeks later...

I got a chance to play with a fairly high end battery tester today. One that asks for the battery spec and prints a nice little report. I haven't looked into it in detail to figure out how it works. It takes less than a second, so I think it looks mainly at peak current (hence the sizeable clamps). The results seem to be consistent with the reading (and my interpretation) from the cheap load tester. I have it for the weekend, so can play around with it a bit more. Not about to invest in one, but it will definitely help when trying to convince a customer to buy a new battery, because of the test result on paper.

Filip

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Two simple battery tests I was told one could perform are, assuming the battery is in the vehicle that is. 
 

1. Resting voltage of the battery. 100% is around 12.5-12.6 volts, less than 12 volts it’s near deaths door. 
 

2. Measuring the voltage while cranking, if it drops below 9v its goosed. 

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