LiFePo battery upgrade question

Teleman wrote:

otrfun wrote:

Teleman wrote:
I ordered a 200AH LifePo battery to replace my two 100AH lead acid batteries and also a compatabile converter which takes care of charging the battery with shore or generator power but what about charging the battery with the alternator? Does the battery's BMS take care of everything and therefore no need to change or upgrade the alternator? Sorry if this is a dumb question!

A BMS is not designed to function as a charger. It is designed as final line of defense to protect the battery.

If you're counting on the wiring to/from the alternator to limit current between your alternator and batteries, you're going to have to choose the proper gauge wire or cable. Too big (physically), may allow excessive current (potentially overloading both the alternator and/or battery). Too little current will result in excessive charge times. There's no magical chart that's going to give you a precise answer. If you've priced copper wire lately, a trial and error approach could get very expensive. Also, if you change size/type of batteries in the future, you'll need to current match the wiring again.

That's one of the upsides to using a dc to dc charger. It will limit current to a precise amount. Sized properly, you're never in danger of overloading your alternator, plus it will properly charge your lifepo4 by precisely controlling the voltage applied to the battery. IMO $200-$300 for a dc to dc charger is a good insurance to protect both your motorhome and battery.

As for using a fuse as an alternator/battery current limiting device, that's not recommended. Fuses are very inexact devices. Some fuses can allow up to twice their current rating before they open. Fuses are primarily designed to protect in the event of a direct short.

What should I be looking for in a DC to DC charger given a 124A alternator and a 200AH lithium battery? I suppose it's possible I may add a second battery in the future as my motorhomes has space for two.

I think pianotuna has been giving you some solid advice. I'd have to agree with his choice of a 20a dc to dc charger to use with his alternator which he described as similar to yours.

Since you're going with a 40a, it's important to keep the voltage drop between the battery/alternator and your dc to dc charger to an absolute minimum (a voltage drop calculator can help determine the proper gauge wire/cable to use). Any significant voltage drop can push the input current to a 40a dc to dc charger as high as 60a. IMO, 60a is way too much load for a 124a alternator--especially if you have a scenario where you have discharged engine and house batteries, headlights on, a/c on, etc. all at the same time. FWIW, the 20a dc to dc charger under the same max load scenario would draw 30a. If you do run into problems with your 40a you do have the option (as pianotuna also mentioned) of resetting a dip-switch on the outside case to drop it into half-power mode so it will operate just like the 20a Renogy.

Lastly, keep in mind the Renogy is not a sealed unit. It has a couple cooling fans that force air (along with any dust and dirt) inside the case. If you mount the dc to dc charger inside a dusty engine compartment this is something to be aware of (along with any water intrusion issues). FWIW, we mounted 40a our dc to dc charger (truck has a 220a alternator) inside our truck camper and made a ~20 ft 2-gauge run from our truck's battery terminals to the dc to dc charger located inside the truck camper. With 40a of charge current we only experience a 43.5a load on the alternator. Larger (physical size) wires/cables decrease the load on the alternator (decreased voltage drop) while still maintaining 40a of charge current.

Good luck with your install!