Alternator Charging LFP from LA via DCDC charger

[lawrence_craig]

Hi there.

I have a 70amp 12V Alternator on my VP engine connected to a LA 100aH starter battery.

If I was to introduce a 12V DC-DC charger to charge a 200aH LFP battery, are there any considerations to be made with the charger in relation to protecting the alternator? For example, if I got a 60amp DC-DC charger, is there risk of the alternator overheating whilst charging? And therefore is it better to go lower (e.g. 30amp)?

Thanks in advance.

N.B. An option here could be to upgrade the alternator to a high output regulated alternator (Balmar) but this is not an option for me because of cost and time reasons.

[two-rocks]

60amp dc-dc will certainly kill your alternator.

The dc-dc has a efficiency loss (15% ish), and the LFE will take it all. Unless you can reduce the output on your dc-dc?

I have a 100 amp balmar that hits factory thermal limits with a 30 amp Victron Orion. But, my engine room has poor ventilation for the alternator and I can safely increase that thermal limit a bit (choose not to).

[Bill O]

Yes, go lower to a 30amp DC DC charger. From what info I've gathered, rule of thumb is 50% of alt. output.

[lawrence_craig]

ah thanks both. It's as I thought. Clears it up, cheers.

[sailorboy1]

Originally Posted by two-rocks

I have a 100 amp balmar that hits factory thermal limits with a 30 amp Victron Orion. But, my engine room has poor ventilation for the alternator and I can safely increase that thermal limit a bit (choose not to).

well you should address that! My 100a alternator is belt reduced to 80a so the belt doeesn't make dust and it ran at that that for hours tyesterday and only got to 88C

[two-rocks]

Originally Posted by sailorboy1

well you should address that! My 100a alternator is belt reduced to 80a so the belt doeesn't make dust and it ran at that that for hours tyesterday and only got to 88C

Rarely (if ever) do I charge with the alternator. Switch for the Orion in the off position. Diesel water / cabin heat - solar does almost all we need.

[sailorboy1]

Originally Posted by two-rocks

Rarely (if ever) do I charge with the alternator. Switch for the Orion in the off position. Diesel water / cabin heat - solar does almost all we need.

If you feel that hitting alternators tremp limit is OK because you rarely use it that is fine with me. Not my alternator!

[two-rocks]

Keep whistling. The temp can go higher. See mainsail and his multiple articles on the matter - and recommendations on increasing the temps. The Balmar factory setting is conservative.

[sailorboy1]

as I said, not my alternator so go ahead and cook it

[newhaul]

Originally Posted by lawrence_craig

Hi there.

I have a 70amp 12V Alternator on my VP engine connected to a LA 100aH starter battery.

If I was to introduce a 12V DC-DC charger to charge a 200aH LFP battery, are there any considerations to be made with the charger in relation to protecting the alternator? For example, if I got a 60amp DC-DC charger, is there risk of the alternator overheating whilst charging? And therefore is it better to go lower (e.g. 30amp)?

Thanks in advance.

N.B. An option here could be to upgrade the alternator to a high output regulated alternator (Balmar) but this is not an option for me because of cost and time reasons.

60 amp dc dc is to big will allow alternator to overheat.
The 50 amp unit would be fine but I would recomend the 40 amp one as its half the cost of the 50 amp one .
I myself have the 40 amp on my 75 amp stock alternator . Plenty of power . If I ever need it a couple hours and I'm full on my 250ah lifepo4. But my solar does 99% of the heavy lifting