High performance battery tester
Anyone who is involved in model making with electrically powered models will sooner or later want to have the opportunity to test their battery packs with a performance corresponding to real use. Many commercially available automatic chargers also offer the possibility to discharge batteries and this very comfortable with adjustable parameters. Unfortunately the maximum discharge power is often limited to a few 10W. So the idea was to combine a commercially available charger with a current amplifier in order to achieve high discharge currents and power.
The principle is quite simple. You take a conventional charger with discharge function (in my case a Robbe I4), measure the discharge current Ib and add an additional discharge current via a current source controlled by Ib with a current amplification of e.g. 19. The battery is now discharged with (19+1) times the discharge current. The functions of the automatic charger remain fully intact, as it did not have any of the additional proportional discharge current.
The circuit is nothing else than a current controlled power source with high performance. The charger is connected to J1 and J4 and the battery to J2/J3. The discharge current of the chargers is measured in shunt R24. The operational amplifier U1 forms a voltage-controlled current source together with the output stages (here 7) connected in parallel. The current amplification can be set to x5 / x10 / x20 / x40 via the DIP switch. The maximum power dissipation is of course dependent on the number of output stages and the cooling. In my sample setup about 100W per power amplifier, so a total of 700W, can be converted. The number of power amplifiers can be extended almost indefinitely.
Here is the cyclic test measurement of my 3S1P3900 from 2011: The DIP switch is set to a gain of 20 and the discharge current on the i4 is 1.5A, so it's 30A.
The whole thing recorded with LogView:
The result: removed charge 170mAh*20 = 3.4Ah, average voltage level 3.64V / cell, voltage difference of the cells <10mV -> good condition!