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Limiting high charge currents?


Thames Bhaji

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I would be grateful for help from any forumites who know about such things. My hybrid diesel / lead acid set up works fine, but I keep toying with the idea of adding lithium. 

 

I have… a 1000Ah, 48v traction battery, a 20kw DC generator, and around 40kw of propulsion motors. I’m typically able to keep the battery above 90% charge, and it only drops that much when we have a day or two without cruising.

 

From what I can gather, adding around 100Ah or more of lithium (roughly the amount I cycle up and down) in parallel with the LA could a) make the LA last longer because they would rarely cycle and b) charge quicker from the generator when not cruising, since the Li take a high current to the end, unlike LA.

 

This last point is my stumbling block. My gen will try to push up to 380A into a battery. This is fine for the LA (since the accepted current soon tails off anyway), but would be too high for lithium, unless the lithium capacity was far larger (and more expensive) than I’d actually need to prevent the LA cycling. 

 

I can limit the current output of the generator, but want to keep it at maximum for when cruising, or for when the LA need charging. So what I really need (I think) is to limit the current going into a modest sized lithium bank to something that is healthy for it, and won’t cause a BMS to just keep disconnecting. I’m imagining that some form of PWM control could do what I need in theory, but don’t know what products (if any) might be suitable. 

 

Any ideas welcome! 

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3 minutes ago, Keeping Up said:

Thinner cables?

 

Whilst thinner cables will limit current to a degree, they will get very hot whilst doing it.

 

I once commissioned a telecomms power plant, where the battery cables were specified too thin and the cables got so hot the insulation was dripping off them!

 

Personally if I were the OP I would leave the system as it is. It works and keeping lead acid batteries between 80% and 100% will ensure a long life due the the low cyclic nature of the operation and the fact that any sulphation will be easily reconverted back.

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1 hour ago, cuthound said:

Personally if I were the OP I would leave the system as it is. It works and keeping lead acid batteries between 80% and 100% will ensure a long life due the the low cyclic nature of the operation and the fact that any sulphation will be easily reconverted back.


Sensible advice I reckon - this is probably an upgrade that would serve to satisfy my curiosity rather than solve an actual problem. It will likely be a long time before the LA’s need replacing, especially since they will still do me OK when they’ve lost significant capacity. Far enough in the future that lithium might be a lot cheaper too…. 

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