B2B

two

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B2B Battery-To-Battery chargers sometimes called DC-DC chargers, allow you to charge your leisure battery from your alternator whilst driving. ... They are capable of providing a full intelligent multi step re-charge of your leasure battery.
 
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How much are B2B battery chargers and who would fit them
 

Tombola

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they range in price from £100+
you can diy fit, plenty of instructions online, and a fair few threads on here

Check you really need one first, by monitoring how much charge you current set up is throwing in already
 
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How much are B2B battery chargers and who would fit them
This member is beatified on this site. Handy for me as in my back yard but does supply with comprehensive instructions or pop down for a trip with a day on his campsite.
 
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I think it is worth saying that a B2B can be used used to charge any one battery from any other one, although they are most commonly used to charge a leisure battery from the starter battery - when the engine is running and the alternator is delivering power.

They are often deemed to be essential on vehicles fitted with a smart alternator - with the B2B activated by the D+ (engine running) signal from the alternator.

Most are capable of providing a full intelligent multi step charge of the connected battery and often have the option to select the battery type and charge rate.

Some will even provide output power when no battery is connected. Often used to run fridges or other equipment from the alternator when the engine is running.

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How would I know if my van had a smart alternator?
I am sure others will have a more technical answer, but my simple check is:-

Plug a voltmeter thingy into the engine cigar lighter socket. A smart alternator will give you the following vaues:-
12.x volts when you are driving (foot on accelerator)​
13.x volts on engine overrun​
14.x volts when breaking.​

If the voltage is substantially constant, then you do not have a smart alternator
 
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I am sure others will have a more technical answer, but my simple check is:-

Plug a voltmeter thingy into the engine cigar lighter socket. A smart alternator will give you the following vaues:-
12.x volts when you are driving (foot on accelerator)​
13.x volts on engine overrun​
14.x volts when breaking.​

If the voltage is substantially constant, then you do not have a smart alternator

So why does it behave like that? What is its motivation?

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So why does it behave like that? What is its motivation?
The answer to that is far too complicated (and, perhaps, too controversial) for this part of the forum.
(and I would suggest, particuarly, this B2B thread)

Search "smart alternators" for full information.
 
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