Alternator+split charge relay V B2B charge rates (19 Viewers)

[Nigel Crompton]

Can someone explain, simply please, how the same device - the alternator - delivers a modest charge rate via traditional means but can be persuaded by a B2B to deliver a thumping 50amps ?

Hmmm ... ?

 

[Pausim]

Split charge relay charging relies on the voltage difference between the engine and leisure battery for charging. It is a crude system. There is often a large voltage drop in the wiring which makes this even worse. A B2B takes the power from the engine battery and optimises the voltage and output to match the needs of the leisure battery. The alternator is programmed to keep the engine battery at a specified level and not to charge other batteries. The B2B separates the two batteries allowing the alternator to do its job properly.

 

[Lenny HB]

You will never get a proper charge or the correct charging profile for the type of battery. Charge rate will be between 5 to 15 amps depending on wiring a bit hit & miss.

A B2B will deliver the correct charging profile much better for battery life and will deliver a decent charge rate. B2B are available in diffent outputs from 15 amp to 120 amp they need to be sized to suit your batteries & alternator.

 

[AdrianChen]

Interesting question!

What actually decides how much charge current a given battery will take, is the difference between the lower standing voltage in the battery (say 12.6v for lead acid), and the higher voltage output of the charger/alternator (around 14.2v with a non smart alternator), and the capacity of the charger/alternator. The bigger the difference the more the battery will take.

My van had a 70amp Split Charge Relay initially with Gel batteries, and a 180amp 14.2v alternator. The net result was a small charge into the batteries once the fridge had used 20amps or so. We now have a 50amp B2B with Lithium's.

If I had connected the empty Lithiums to the alternator directly with a big fat 120mm²+ cable the alternator would supply its full 180amp output to the batteries which can accept up to 300amps charge. In practice it would get pretty hot and throttle back it's output, and/or burn out at some point!

If I had left the existing Split charge relay in place, and the existing 16mm² wiring but attached the same empty lithiums, the charging rate would be much reduced because of voltage drop in the cables theoretically around 4volts in my case, so the alternator would be putting out 14.2v, but the battery would see 10v ish, and not charge, in practice the voltage drop would be lower because the batteries would accept less charge as the voltage dropped, until it all balanced out, so for example if the batteries were seeing 13.2v they would perhaps accept 50amps at which point voltage drop would be around 1volt. In practice the cables might get dangerously hot, and the SCR would likely burn out pretty quickly.

So what limited the original system to a modest output, was the gel batteries did not have a huge acceptance rate given the resistance in the cables (including the SCR itself) kept the voltage at the batteries end somewhere not much above 13.5v(ish).

So the b2b was put in to manage the max currents in the system, thus protecting the alternator and to provide the correct charge profile to protect the battery. To install the B2B safely/correctly the cable sizes were increased to suit. If all I had wanted was a higher charge current then bigger cables and a higher capacity SCR would have done it, but potentially risking the alternator going pop.....

Crap at explaining stuff in writing, so hope this helps!

 

[Cal54]

Excuse my ignorance as I’ve never had a B2B installed. My current little van is my only vehicle and therefore is in use most days of the week, even if only to the local shops. Is a B2B still recommended in these circumstances?
Thanks for any advice.

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