Saturday, 2 February 2013

Another graph

Went out for a drive today. Here is a graph of voltage and current over time. I've tried to scale it so that 0 Amps is at the same position as 12.8V on the y-axes.

Current and Voltage over time whilst driving
The interesting thing is that the Sterling B2B is only putting in 16A of charge when driving. The charger is rated to 50A and the alternator is 75A, so I should be able to get some more current in. Indeed, when I switched on some load the battery with the charger running, the battery was still getting a constant 16A, which means the charger has current to spare, but the battery can't take it.

I think I would expect a battery to be able to take more than 16A charge rate. The general recommended rate for charging a lead acid battery is 20% of its Ah rating. ie. for a 115Ah battery, it should ideally be charged at 23A. So I would expect the battery to be able to take more.

I did notice that my Sterling B2B charger has been on the 'Gel' setting all the time I've had these batteries (2.5 years). The charger actually has a 'Sealed Lead Acid' setting, which is more appropriate. According to the manual the Gel setting has a slightly higher float voltage, but the main difference is it's bulk stage duration says '12-24 hours' versus '1-12 hours' for the SLA setting.

So I'm now wondering if my batteries have been being undercharged all this time. Thinking about the general pattern of charge on my batteries they are either charged by the Sterling B2B when driving or by the solar panel when stationary. I've long suspected that the Sterling gets confused by the solar panel. When we start driving, the Sterling B2B charger measures the voltage on the battery bank and from there works out how long the bulk charge stage should be. Since the solar panel is always on, the batteries read a higher voltage then they really are. So I think the Sterling has been running the bulk cycle too short. This combined with being on the Gel setting maybe means they have never reached the end of the bulk stage properly.

1 comment:

  1. You could use your arduino and a relay board to steer open the solar for 30 seconds when the ignition enters the car. So the charger has time to measure a sleeping battery (although it will take more time to have a battery at rest.)