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Battery Care

Published on: 26 July 2017
Country: United Kingdom
Race category: IET Formula Goblins Ages 9 - 11, IET Formula 24 Ages 11 - 16, IET Formula 24+ Ages 16 - 25

Electrics

Even brand new batteries require attention in order for us to get the best possible performance out of them.

This is not the only way to get the best out of batteries, but this is what we found has worked.

After receiving brand new batteries this is the process we have followed:

  1. Allow them to reach normal room temperature.
  2. Charge until your charger says they're fully charged. We use Optimate4 chargers but virtually all smart chargers will follow pretty much the same regime and declare the battery charged at the same voltage.
  3. Partially discharge the battery for 5 minutes at 20A - 25A.
  4. Charge again until complete, this may take longer than you expect after such a short discharge. I have no idea why this should be but the partial discharge seems to be a critical part of battery preparation.
  5. You should then cycle them three times on the bench before using in a race as a battery's capacity seems to increase over the first few cycles. Make sure you don't discharge to below 10.5V, we stop the tests at 11V. Allow the battery to recover for an hour after charging and discharging. By "cycling", I mean charge the battery, then discharge (down to 11V under load), then recharge fully.

Bench cycling of batteries needn't be complicated, all you need is a load, a switch, a multimeter and a stopwatch. Four or five 12V 50W halogen bulbs in parallel make a suitable load of about 17A or 21A respectively, so connect the battery to the bulbs and with the stopwatch time how long it takes for the battery voltage to fall to 11V then switch off immediately. Whoever is watching this test needs to be extremely vigilant to ensure the voltage doesn't drop too low and damage the battery, however if you have any electronics knowledge it is quite feasible to automate the discharging process.
 

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This rats nest of wires is our battery test rig, a motor controller board on the right and a picaxe board on the left. Originally the picaxe board provided motor current control, datalogging, telemetry, GPS, and driver display in one of our earlier cars. Here it measures voltage and current which it sends via a serial link to a laptop for logging, once the voltage drops to 11V the motor controller board is switched off.

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The motor control board switches the power to these 50W 12V halogen bulbs.
 

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This is a Hall Effect current sensor that we buy from Devicecraft,a small company in the USA, very simple to use with a Picaxe, Arduino, etc, inexpensive and usually only take a week to get here.

It is very important that you put fully or partially discharged batteries on charge as soon as possible. You may not feel like it after a long day's racing but if you don't your batteries will suffer a small but permanent reduction in capacity. This has always been a major problem for us because we live on the north coast of Scotland and it can be more than a day before we get home from most races, so I speak from experience when I say your batteries will lose capacity.

Temperature. Greenpower regulations allow batteries to be at a maximum of 25C, you should aim to be close to this limit as a battery's capacity will be reduced when discharged at low temperature. If your batteries have been in a cold trailer overnight before the Final in October they could well be lower than 5C and have 15% less capacity than they would at 25C. Teams use all sorts of things to keep batteries warm, reptile heating mats, Peltier effect hot/cool boxes, electric blankets, there are lots of options.

Time

After a few weeks of inactivity the batteries seem to go slightly dormant and a show a 5% to 10% reduction in capacity, so if it's been more than a couple of weeks since your last race it's worth giving the batteries a cycle to wake them up.

The big question is how much power should you be able to get from your batteries? 17-18A for 90 minutes seems to be a reasonable figure for the Yuasa REC36's currently being supplied by Greenpower.

- Gary Coulden-Smith

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