Good rpm for charging

[leeboy]

Hello what would be a good rpm for charging two 460A lithium batteries off a beta 43 with a standard alternator l have also got two 12/212 50 A battery to battery chargers. Charge in gear or out of gear ? 

Thanks for taking the time to read 

[blackrose]

Others will know better then me but it will vary depending on your engine/alternator setup. I think you need to raise the rpm slightly above idling until the voltage increases to its maximum. If you keep increasing revs you won't achieve any increase in voltage so at some point you're just wasting fuel. It's about finding that optimal point. For me it's about 900rpm (Isuzu 55).

Edited December 13, 2025 by blackrose

[Alan de Enfield]

9 minutes ago, leeboy said:

Charge in gear or out of gear ? 

 

You must only run your engine out of gear when moored (It is one of the 'rules' in your licence application)

Obvioulsy you can run the engine in gear when cruising !

 

The ideal revs are when your battery monitors show maximum charging - this is normally a fair bit above tick over.

[LadyG]

16 minutes ago, leeboy said:

Hello what would be a good rpm for charging two 460A lithium batteries off a beta 43 with a standard alternator l have also got two 12/212 50 A battery to battery chargers. Charge in gear or out of gear ? 

Thanks for taking the time to read 

Err, I don't think you should be charging in gear when moored  is this a UK canal / river?

Generally speaking you are best to charge "Lithium" up to 85%, others will advise.

Your set up seems unusual.

For best answers more info needed ...

Edited December 13, 2025 by LadyG

[Jen-in-Wellies]

Out of gear, because charging in gear undermines banks. If you are in the middle of a marina, far from the bankside and you don't mind annoying the neighbours, then in gear might be acceptable. 

1200 to 1400rpm. Slower puts more load on the engine. Faster won't gain anything and wastes fuel. 

When you say standard alternator, is that one, or two alternators? If two, I'd consider knocking back the programmed charge current of the B2B for the engine start batt alternator, as 50A may overheat the alternator. The standard house alternator on a Beta 43 is happy having 50A taken for hours. I am also assuming you have a lead acid batt across each alternator. 

[b00ke23]

I think Beta stipulates at least 1200rpm minimum for battery charging. 

[IanD]

17 minutes ago, b00ke23 said:

I think Beta stipulates at least 1200rpm minimum for battery charging. 

Yes, IIRC there's a warning IN RED CAPITALS in the manual saying this, also nowadays often a sticker in the engine bay saying the same...

[leeboy]

1 hour ago, Jen-in-Wellies said:

Out of gear, because charging in gear undermines banks. If you are in the middle of a marina, far from the bankside and you don't mind annoying the neighbours, then in gear might be acceptable. 

1200 to 1400rpm. Slower puts more load on the engine. Faster won't gain anything and wastes fuel. 

When you say standard alternator, is that one, or two alternators? If two, I'd consider knocking back the programmed charge current of the B2B for the engine start batt alternator, as 50A may overheat the alternator. The standard house alternator on a Beta 43 is happy having 50A taken for hours. I am also assuming you have a lead acid batt across each alternator. 

Thanks l don't run my engine in gear, very helpful cheers merry Christmas 🎄 

[Alan de Enfield]

1 hour ago, leeboy said:

Thanks l don't run my engine in gear,

 

 

I bet you don't move very far !

[MtB]

1 hour ago, Alan de Enfield said:

 

 

I bet you don't move very far !

 

Hopefully at least a mile a fortnight!

 

 

But getting back on topic, assuming 1) the OP's two B2B chargers are 50A each, and 2) there is only one alternator charging the LFPs, 3) both batteries are being charged simultaneously, and 4) the B2Bs are say, 80% efficient then when running together they are capable of sucking 125A out of the one, possibly overloaded, alternator. If the drive belt is a multi-Vee then this is marginally OK from the POV of belt wear but if it is a conventional V belt then this needs dialing back quite a lot in the B2B software. Dragging anything more than about 75A from a mono-V belt alternator is asking for trouble with belt wear and/or slippage, I'd say.

 

If its a poly-vee belt then depending on the make and model of alternator (no such thing as 'standard' alternators) it might be good for the 125A but overheating on long charges might still be a risk. It would also be a Good Idea to have a LA buffer battery before the B2Bs to guard against alternator damage when the LFP BMSs disconnect when reaching fully charged. 

 

 

 

 

 

 

Edited December 13, 2025 by MtB

[cheesegas]

13 hours ago, MtB said:

But getting back on topic, assuming 1) the OP's two B2B chargers are 50A each, and 2) there is only one alternator charging the LFPs, 3) both batteries are being charged simultaneously, and 4) the B2Bs are say, 80% efficient then when running together they are capable of sucking 125A out of the one, possibly overloaded, alternator. If the drive belt is a multi-Vee then this is marginally OK from the POV of belt wear but if it is a conventional V belt then this needs dialing back quite a lot in the B2B software. Dragging anything more than about 75A from a mono-V belt alternator is asking for trouble with belt wear and/or slippage, I'd say.

 

If its a poly-vee belt then depending on the make and model of alternator (no such thing as 'standard' alternators) it might be good for the 125A but overheating on long charges might still be a risk. It would also be a Good Idea to have a LA buffer battery before the B2Bs to guard against alternator damage when the LFP BMSs disconnect when reaching fully charged. 

 

Agreed on the limits of a single V belt, but a polyvee has a much higher power transmission ability. 120a at 12v along with some headroom to account for the alternator's own inefficiency is fine - 2kw input on a 4pk belt is well within spec. Travelpower units run at 3kw output for example, although they're a fair bit more efficient than a 12v alternator.

 

 

@leeboy - Assuming it's a single alternator on a Beta 43, the standard starter alternator is only 40 amps on a single V belt, but you can specify a 70 amp or 100 amp (I think, could be 120 amp) model too, both of which come with a polyvee belt setup. Do you know what you have? A photo would be helpful. As @MtB said, you may be overloading the alternator.

[nicknorman]

53 minutes ago, cheesegas said:

Agreed on the limits of a single V belt, but a polyvee has a much higher power transmission ability. 120a at 12v along with some headroom to account for the alternator's own inefficiency is fine - 2kw input on a 4pk belt is well within spec. Travelpower units run at 3kw output for example, although they're a fair bit more efficient than a 12v alternator.

 

 

@leeboy - Assuming it's a single alternator on a Beta 43, the standard starter alternator is only 40 amps on a single V belt, but you can specify a 70 amp or 100 amp (I think, could be 120 amp) model too, both of which come with a polyvee belt setup. Do you know what you have? A photo would be helpful. As @MtB said, you may be overloading the alternator.

Later Beta 43s (by which I mean in the last 13 years or perhaps older) have at least a 175A alternator as standard. 120A is about right for max continuous output without getting too hot. And as said, around 1200 -1300 rpm for charging.

[JoeC]

my Beta 43 from 2001 came with a domestic alternator of 95 Amp.

[nicknorman]

3 minutes ago, JoeC said:

my Beta 43 from 2001 came with a domestic alternator of 95 Amp.

My one from 2010 came with 175A. I am not sure when it changed.

[Jen-in-Wellies]

The paperwork for my 2007 Beta 43 engine says 150A for the house alternator. 

[waterworks]

On 13/12/2025 at 15:20, Alan de Enfield said:

 

You must only run your engine out of gear when moored (It is one of the 'rules' in your licence application)

Obvioulsy you can run the engine in gear when cruising !

 

The ideal revs are when your battery monitors show maximum charging - this is normally a fair bit above tick over.

Did you put the word " rules" in speech marks because you doubt ( rightly) that it is not legally anything to do with the licence application and CRT are lying   ?

[nicknorman]

12 minutes ago, waterworks said:

Did you put the word " rules" in speech marks because you doubt ( rightly) that it is not legally anything to do with the licence application and CRT are lying   ?

I can't answer your question, however whilst it is nothing to do with the licence application, a by-law does make it illegal to run the engine in gear when tied up. By-law number 38 in the 1965 bylaws which as far as I am aware are still valid. Whether breaching the bylaws is a reason to revoke a licence is another matter. Probably not, even though it should be for repeat offenders. And also of course it is a selfish and stupid thing to do.

[MtB]

39 minutes ago, nicknorman said:

Whether breaching the bylaws is a reason to revoke a licence is another matter.

 

 

I'd say no it isn't. Bylaw breaches are nothing to do with licencing IIRC. 

 

(Also IIRC) a bylaw breach attracts a fine of £100 on conviction according to Nigel Moore, with BW and subsequently CRT being the body with power to mount a prosecution. 

 

No doubt this is not quite right but rhymes with the truth and Alan De E will post 150 pages of copy-N-paste with one paragraph in it somewhere, that explains the true position