'Safe' parallel charging boards

My "battery checker" is only good for a guide. I estimate that the checker is around 10 -15% out. I do find that the milliamps put back in don't compare favourably with the "checker" either. Of course it does depend on the quality/accuracy of components inside the checker and circuitry. I rely on my i306b charger for real cell voltages. And yes, I have verified using a calibrated voltmeter.