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One of our cars recently had problems starting on a ~70°F day, after having the headlights on for 10-15mins with the engine stopped, post a 20 mile drive.

The decision was to replace the battery, which was done and we have had no issues with the car starting/running since. However, I am interested in the state of the "old" battery.

What method(s) would the community suggest for someone with a moderate understanding of electronics and electrics, to test this?

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This question came from our site for electronics and electrical engineering professionals, students, and enthusiasts.

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    If it really means a lot to you, the best solution would be to pick up a "Battery Load Tester" for about 25-40$ US and hook it up to the questionable battery. To test a battery, it really needs to be put under a load. How old was the original battery? A lot of times, I've found just bad/questionable connections when I thought the battery was dead. – R Drast May 7 '15 at 16:30
  • A new battery always seems to solve the problem, but the problem might not be the battery. Look carefully for poor crimping of lugs to wires, they get hot while driving. – tomnexus May 7 '15 at 16:47
  • @RDrast - Sorry, I should have stated that I was hoping for a solution without needing to buy "specialised" equipment. I am unaware of the service life of the battery, nor it's age. Battery has already been swapped, but will keep bad connections in mind if I test it still to be 'good', and put it back into use. – user66001 May 7 '15 at 17:30
  • @tomnexus - I would have assumed that this kind of issue would have been the same/worsened with the new battery, had it been the issue? – user66001 May 7 '15 at 17:31
  • If there is a problem, it will probably happen again, yes. But for a few days or even weeks, the fresh new battery will make everything will go well. Just be very alert to any "battery issues" in the next few months. – tomnexus May 7 '15 at 17:45
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Charge it completely full, until it floats at 14.8 V or whatever is recommended.

Then hook it to a modest load, perhaps a 1 A tail light, and plot the voltage as it discharges. Stop when you get to 8 V and immediately recharge it so you don't do too much damage.

This should give you an idea of the remaining capacity in the battery.

Read the whole of batteryuniversity.com and also dig on the internet for datasheets of similar batteries, for their discharge curves. And read on StackExchange, in fact, the right hand column is full of similar questions!

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I used to run several vans for work and batteries were always a problem. I found that an hydrometer to check on them was very useful. It will show you exactly which cells are 'bad' and whether they could be recovered.

This sort of thing: Mighty Max Battery Hydrometer

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