Help sizing Renogy DC to DC charger for 110amp alternator

[caddy camper]

Hello there i have a caddy 1.9tdi 2009 micro camper i've built for a trip to spain in january for 2 months
I've just got a renogy 100ah lithium battery and 20a DC to DC charger in the black friday sale but now thinking should i have gone for the 40a charger instead as obviously it will charge the battery twice as fast.
the renogy battery has a 100a BMS and has a Maximum continuous charging current of 50a maximum discharge 100a
the charger with be under the passenger seat around 3m cable length

Would the 40a charger be ok for the caddy alternator rated at 110a?
i don't want to over work the alternator to much but if it can handle the 40a charger id go for that one.

the limited research i've done suggests to match your DC to DC charger to the alternator idling amperage
i'm not 100% but what i can find the alternator idles at 50% its rated output so my caddy alternator should idle at 55a.

Many thanks james

 

[pappajohn]

The 40a charger is a maximum rate. The battery will only take what it needs just like the alternator will only put out the current needed, not 110a continuous.....that's only the maximum output it's capable of

 

[caddy camper]

thanks for the reply pappajohn
do you think the 20a charger i have will be ok or would you recommend upgrading to the 40a?
i'm going on a road trip to spain jan and feb and may stay in the van 3/4 nights at a time i have a small aplicool 15l 12v compressor fridge 4 LED downlights and will be charging a phone tablet and sometimes a laptop oh an i have a 2kw diesel heater too.
i have a 100w dokio folding solar panel to top up when possible.
the 20a will be fine when im going from A to B but worried it may lack when in staying somewhere for a few days although i expect to be driving most days even when camped somewhere
many thanks james

 

[bigtwin]

The 40a charger is a maximum rate. The battery will only take what it needs just like the alternator will only put out the current needed, not 110a continuous.....that's only the maximum output it's capable of

True, but only really relevant to lead acid batteries as their internal resistance varies significantly with the SoC. This is not particularly relevant to the OPs case where he’s using LiFePO4.

Ian

 

[pappajohn]

True, but only really relevant to lead acid batteries as their internal resistance varies significantly with the SoC. This is not particularly relevant to the OPs case where he’s using LiFePO4.

Ian

Missed that bit ...must take more notice.

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[Lenny HB]

The 40a charger is a maximum rate. The battery will only take what it needs just like the alternator will only put out the current needed, not 110a continuous.....that's only the maximum output it's capable of

A lithium will gobble up as many amps as you can throw at it.

110amp is a pretty small alternator these days, I would stick with the 20 amp, better to have a not fully charger leisure battery than a burnt out alternator.

 

[caddy camper]

thanks Lenny HB i thought i may be pushing the alternator with the 40a charger 20a may be better for the battery in the long run.
they do an upgraded 140a alternator but its not a big enough jump in amps to justify.
i'll stick with the 20a charger save sending it back and getting bigger cable and fuses
i was going to put a switch in-between the D+ to ignition live so i can turn it off on long journeys or when not using it also use as a daily driver/work van
thanks everyone for the input

 

[autorouter]

i was going to put a switch in-between the D+ to ignition live so i can turn it off on long journeys or when not using it

No harm in doing that, but it's not really necessary. The B2B will stop charging when the battery is full, so the amps will drop to zero.

 

[caddy camper]

thanks autorouter much appreciated save me a job