Battery tester

edited October 2011 in IET Formula 24
Lots of comments about battery issues on the 2011 final thread.
We made a discharge tester using components from RS and a "hours elapsed" timer off ebay (£5 I think). I will post details, RS stock numbers, schematic and pictures so others can replicate.
The device discharges the batteries at 18A through power resistors until a 10.6V threshold is reached then switches off, leaving zero current draw on the batteries and the discharge time on the timer.
This will enable you to identify failing batteries, pair them up & choose best batteries for particular race strategies. The circuit is simple, 1 relay, 1 transistor, one push button, a zener diode & a big slab of metal to heatsink the resistors.....(OK there's a diode in there too....)
Others have used headlights as load resistors in the past, only problem with these is that the resistance is unknown & (massively) variable.


  • Bob. We would love one of these! Thanks for the offer! Do you want my email? CR
  • i too would appreciate the details if possible. thanks

  • Hi Bob - any schematics and details for this will be greatly appreciated. Like many others we had one pair of batteries that were not as effective as the rest but its difficult to determine why.

    Good to catch up with you again on the weekend.

  • OK, getting stuff together: - I want to be sure it works because it's actually a simplification of the first one we made, so give me a day or two. CR - I'll link everything on here so you shouldn't need any more data.
  • whswhs
    edited October 2011
    Remember what we were talking about at the final Bob, about passing on expertise? And credit to you for so doing........ again. ;-)
  • I can let you know how to make a good cup of tea?
  • Ooh, yes please! I've been using the headlamp method in conjunction with a Watts Up meter but it's very difficult to get the correct current draw so always having to extrapolate which is non too accurate. Many thanks and will look out for this BobC as we had duff batteries at the Final too.

  • my chocolate fudge cake has won a trophy at our local village show - i'll can let you have the recipe if you want- goes well with a cup of tea.
  • edited August 2016
    OK I'll start with some parts for the shopping list....
    The idea with this thing is that you wire it onto a full battery, press a button to start the discharge then walk away - it discharges at near the F24 rate until the battery goes down to 10.6V then it stops discharging and the timer tells you how long it's taken. You can go back to it a day later if you want. The battery is not damaged because the discharge stops with an on-load voltage of 10.6V (after switch off this will spring back up to 12.1V or so).
    I found a suitable timer on ebay for about £7.50. it's called a "mini time accumulator" and you want the "volts free" one with display description "0-99h59m59s" e.g.

    Admin edit: The above timer no longer available. Try:

    The circuit can be built on stripboard. Below I give the RS components stock numbers of some of the important parts:
    relay (418 6170) cost £2.34
    zener diode (10V) (812 465) cost 26p x5 or any 10V zener
    diode (544 3480) cost 1.2px20
    2 x any small signal diode e.g. 1N4148
    transistor BC557 (545 2260) cost 10px5 or any small signal PNP
    power resistors (252 2906) cost £3.95 - you need 2 of these, it's a 100W 1.5ohm resistor
    You'll also need a pushbutton switch and a couple of small resistors at 100ohm and 220ohm.
    Depending on your heatsink you will probably want a small 12V fan as well. you're pushing 200W into the heatsink, it's going to warm up ;^)
    Oh yes - have you noticed you'll need a heatsink of some description...
    I'll post links to some photos and a schematic shortly ;^)
  • Heatsinks: you need quite a biggy.... I forget about these 'cos I can just grab one from the scrap bin at work......
    No matter, ebay is your friend...... a 200mm length of this should do (for your nice little battery fan heater....)
  • BobC.....thanks for all of the above. Been to the ebay site for the timer but which one is the 'volts free' unit you mentioned? Can't find any reference to that except in the blurb where it says no external power source is needed.
    Is that what you mean?


  • edited October 2011
    right now it's the 2nd one down - it has to display 0-99h59m59s and it has to NOT mention 110-240Vac!! The text will say something about not needing an external power supply,
    I've stuck one together this morning & it works so I'll take a couple of pictures & link 'em on here - might even do a youtube vid of using the thing later (when my 2nd timer comes from fleabay....)

    I debugged mine by disconnecting the big load resistors and using a bench PSU on low current (<100mA) to try it out. With the PSU on 12V the relay clicks when you press the button & stays shut til you reduce the volts to 10.9V, then it clicks open. ALways a gamble trying to debug with a SLA as the PSU, those things have no conscience & will fry your mistakes in a flash!!!!
  • edited July 2016
    OK here are some pictures: complete discharger module
    discharger photo discharger.jpg
    the control circuit on veroboard (stripboard)
    dischargercontrol photo discont.jpg
    and, most importantly, the circuit diagram
    dischargerschematic photo dischsch.jpg

  • On to this and thanks a lot for all the information.. Only glitch is that I can't find any reference to the RS stock numbers you give on their website. I've tried putting them into the 'search' but they are not recognised. I can probably find the items from your description but I just wondered where they came from as I want to make sure I get the correct components, such as the relay 6170.

  • Hi Chris,
    the search should work - look for the whole stock number e.g. 418 6170 or 4186170 and it should take you straight to it - I'm afraid this site is messing with the original post. I'll edit that to take the hashes out.....
  • Got you.......I didn't read it as on number sequence and thought the two were different references.......don't often use RS so didn't recognise it! Thanks again.

    What would you need to do to the circuit if you wanted to apply a different current draw value? I understand this is for 18 amps which makes perfect sense for most cars but perhaps one might want to try a higher rating with a super aero car that weighs next to nothing!

  • Depends chris - more amps need thicker wire, perhaps a bigger relay, bigger power resistors (of lower resistance value), bigger heatsink, bigger fan.... What I have described here does the job of matching and finding dud batteries - I can't really see the point in changing it.
  • Thanks Bob - I have ordered the counter and located quite a few of the other electronic components and now have to wait for postal/delivery services. The heatsink and fans are a bit expensive so I am going to look through old electonic units our IT department have kicking around. It looks like you also used heatsink paste between the power resistors and the heatsink base which will help.
    When we charge our batteries they typically get up to 13.6Volts and after settling they end up at 13.2Volts. During this years racing they showed 12.6V after we got back to school. We did find that there was a noticeable difference in the longevity of the batteries power delivery between the three matched sets that we had. What would you expect a good battery would show in Volts before a race and again after they have cooled after the race?

  • Phil, the voltages you mention sound typical for what I'm seeing too ;^)
    Don't scrimp with the fan, the one in my picture above is quite a chunky 60mm square job and is only just big enough, a windier one would be better - in fact I'd recomment an 80mm square one.
  • Ok - good cooling it will be! I don't do electronics on a regular basis and can't imagine what 200 watts would feel like. Best be safe and troll ebay for something appropriate.

  • search "80mm fan" on ebay, you'll find hundreds for a couple of quid.... Get one that's chunky & I'd go for "noisy" so you know when the test has finished.... ;^)
  • Fan and counter have arrived but have not yet ordered the heatsink; assembling the circuit willl start next time I am in school.

  • Here's a quick video of the complete tester in use

    enjoy...... ;^)
  • Nice work Bob - I take it even though it was not such a good battery, that it wasn't fully charged before the discharging test to only last 17 minutes. How long would you expect a fully charged - good battery to last on your discharge unit?
    Im back in tomorrow and can't wait to get started with our discharger / timer. There will probably be more questions that I will need to ask but with the circuit diagram I think I can replicate what you have so superbly shown us.


  • That was a completely bazooka'd battery; good ones go between 1:28 and 1:36. Some hints for making such a circuit work...
    1) check each node with an ohm-meter i.e. check every component lead is connected to exactly what it should be AND NOTHING ELSE
    2) try it out on a bench power supply, not on a battery - a battery will supply hundreds of amps into an accidental short circuit
    3) bware you don't short things when you mount the controller board, e.g. via the mounting screws
    4) think about what are the high current paths & use suitable wire cross section. I have just used the wires I had to hand, but you can probably identify ~20A paths, ~10A paths and flea power paths (<100mA). If you use 1mm^2 wire for 20A it WILL get warm
    just for general interest, that particular battery was killed by leaving it completely empty for 4 months; it's perhaps more surprising that some other batteries actually survived the experience...
  • Apologies to the old hands on this forum but would also like to rake up this old post for the Bryanston guys for a most excellent battery tester from BobC. We have made two of these now and find them invaluable for checking our battery health. They make an excellent project in the Greenpower sessions, probably a couple of hours to make once you have all the parts.
  • Appreciate the comment :)
    I made another one of these a month or so back; the toughest thing to find is the time counter thing. Ebay still does them if you search for "time counter accumulator". There are usually a few on there by FENG YUAN but mostly these are "0-9999.9" which is the wrong sort of display. you want the ones described "Range 99H59M59S LCD Dispaly No-voltage Required Time Counter Accumulator" or something like. 0.1hr resolution is not really good enough.
  • Handy Hint No. 36....

    When you discharge a fully charged battery, through Bob's device, you are assessing its capacity to store electricity. Over time, that capacity drops, due to chemical and mechanical changes inside the battery. If you have treated your batteries well, they will last a long time, but leaving them in a discharged state will result in rapid loss of capacity. If that happens, there is a chance of some recovery.

    Find yourself a voltage limited and current limited power supply. Set the voltage limit to 15 V and the current limit to 1 A. Charge the battery normally then connect the power supply to the battery and leave it for about 6 hours. The effect will be to equalise the charge in all the cells -those that charge up first will produce hydrogen gas but the 1 A charging current will not cause a problem. After the equalisation charge, recheck the capacity. You might be pleasantly surprised.

    My own car battery has had this treatment and is still going well after 7 years. I've also 90% 'recovered' capacity from a battery that was down to 8 Ah from around 60 Ah. On some batteries I have applied up to 18 V to good effect. If you try this, don't push too much current into the battery else it will heat up excessively.
  • Note that the YPC33-12 (black) Greenpower batteries have a very sharp voltage/current "knee" at about 14.7 Volts (actual voltage dependent on temperature). Above this voltage they will continue to take quite a lot of current and gas a lot even when fully charged. We've had chargers not stop charging due to this damaging the batteries. The main problem being that there ain't much water in these VRLA batteries to lose through gassing so although overcharging them a bit as OldTimer states helps fully top them up, make sure you restrict any over charge time as OldTimer states (I would say < 4 hours ?) or limit the current to less than 100mA. We think the newer REC36-12 batteries (grey sides) don't have quite the same issue, but we haven't had so much experience of these.
  • I haven't seen the same issue with the REC36-12 batteries, although as with you, I haven't performed nearly as much testing on them. I saw YPC33-12s still taking 2A+ on a power supply at 14.8V when fully charged, and getting very hot. Since then I've never charged them on a power supply above 14.5, and then used a CTEK to top the last bit off.

    We've caused ourselves issues by charging multiple YPC33-12s in parallel on the C-TEKs, as the current never drops low enough for it to move on from the 'hard charging' phase.
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