LiFePo battery upgrade question

[Teleman]

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!

[Teleman]

Since both my staring and house batteries are under the hood I need to figure out where to mount the DC to DC chager.

[Teleman]

pianotuna wrote:
Being a class C person--under the hood = heat. *wiping the egg from my face*

I will never own Li batteries (unless Lithium Titinate drops in price), because I camp at -30.

Is there a reason to not use the OEM battery compartment?

That is the OEM battery compartment! They are on a shelf under the small hood and close to the starter battery.

[pianotuna]

Being a class C person--under the hood = heat. *wiping the egg from my face*

I will never own Li batteries (unless Lithium Titinate drops in price), because I camp at -30.

Is there a reason to not use the OEM battery compartment?

[Teleman]

pianotuna wrote:
Teleman,

You Li batteries will not "like" life under the hood.

DC to DC are not happy about under the hood either.

I have a similar alternator. I deliberately chose a 20 amp DC to DC unit from Renogy.

If I were buying again, I might go to 40 amps, but only because it has a "link" that when removed puts it back down to 20 amps.

https://www.renogy.com/12v-20a-dc-to-dc-on-board-battery-charger/


Victron makes a similar more expense (but probably more reliable) unit.

Can you explain why? Bear in mind under the hood in a Class A is isolated from the engine. The only problem I see is moisture when it's raining.

[pianotuna]

Teleman,

You Li batteries will not "like" life under the hood.

DC to DC are not happy about under the hood either.

I have a similar alternator. I deliberately chose a 20 amp DC to DC unit from Renogy.

If I were buying again, I might go to 40 amps, but only because it has a "link" that when removed puts it back down to 20 amps.

https://www.renogy.com/12v-20a-dc-to-dc-on-board-battery-charger/

Victron makes a similar more expense (but probably more reliable) unit. I suspect that if I had a "do over" I might go Victron.

[Teleman]

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.

Something like this? My batteries are under the hood. Can a device like this be mounted there as well, exposed to the elements? I'm assuming it needs to be close to the batteries connected with heavy cables.

https://www.amazon.com/Renogy-Battery-Batteries-Multi-stage-Charging/dp/B07Q4SVX3M/ref=sr_1_2_sspa?crid=V0110A4QZOUL&keywords=renogy+dc+to+dc+charger&qid=1636383357&sprefix=renogy+dc+t%2Caps%2C314&sr=8-2-spons&psc=1&spLa=ZW5jcnlwdGVkUXVhbGlmaWVyPUEyWUJBNEJJS1JRQ01HJmVuY3J5cHRlZElkPUEwODczNDQ3VFBMOFJEWFIyOFBYJmVuY3J5cHRlZEFkSWQ9QTAyMjY3NzExR1Q1Wk5YWUdMQVUyJndpZGdldE5hbWU9c3BfYXRmJmFjdGlvbj1jbGlja1JlZGlyZWN0JmRvTm90TG9nQ2xpY2s9dHJ1ZQ==

[Teleman]

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.

[Gdetrailer]

otrfun wrote:


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.

Fusing as a "current limiting device" on a regular basis is a wrong application for a fuse.

However, it IS recommended to ALWAYS include a fuse, fusable link or circuit breaker in all power sources wiring especially when dealing with high amperage sources.

The fuse/fusable link or circuit breaker is there to protect your wiring in case of catastrophic short on the wiring.

Fuses/fusable links or breakers do not have to be "precise" and there are different types which have intentional delays for specific types of loads. But even so, are there to protect your wiring from getting hot enough to melt wire insulation and cause fires.

[otrfun]

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.

[pianotuna]

Teleman,

Never be sorry you asked a question.

The reason for the DC to DC charger is to limit the current being sent to the LI battery bank. Duty cycle on alternators is often 1/3. For example, a 120 amp alternator is intended to only produce 40 amps continuously--except for brief periods of time.

A side benefit is that you will be in control of how much charging is being done. Li will charge fast--but for maximum cycle life, slower charging may be best.

Here is a fairly new video about care: https://youtu.be/xRUQCxWGyfQ

[Teleman]

Teleman wrote:

time2roll wrote:
Dig in and find a part number.
Call Airstream.
Ask a workhorse chassis repair facility.
If you post the year, model etc someone might know.

OK thanks for your help.

Google search says 124amp for my model year (2000)

[Teleman]

time2roll wrote:
Dig in and find a part number.
Call Airstream.
Ask a workhorse chassis repair facility.
If you post the year, model etc someone might know.

OK thanks for your help.

[time2roll]

Dig in and find a part number.
Call Airstream.
Ask a workhorse chassis repair facility.
If you post the year, model etc someone might know.

[Teleman]

time2roll wrote:

Teleman wrote:

time2roll wrote:
OEM charging, boost start, bi-directional relay. Possible other options I don't know.

Yes the Victron shunt will tell you immediately if the amps are excessive. Excessive is primarily based on the wire and fuse rating.

Pardon my ignorance but what numbers would be excessive? I saw a YouTube video where the alternator overheated charging a lithium battery at a low rpm where it was passing a lot of current but not turning fast enough to cool itself with its internal fan

Yes that video is a bit unrealistic so I recommend measuring your real situation.
I read maybe 105 to 120 amp alternator is common with some old ones at 65 amp and some replacements at 200 amp. Depending on the battery size I would look for no more than 30% to 50% of alternator rating.

Thanks for your patience. How do I know my alternator’s rating?

[time2roll]

Teleman wrote:

time2roll wrote:
OEM charging, boost start, bi-directional relay. Possible other options I don't know.

Yes the Victron shunt will tell you immediately if the amps are excessive. Excessive is primarily based on the wire and fuse rating.

Pardon my ignorance but what numbers would be excessive? I saw a YouTube video where the alternator overheated charging a lithium battery at a low rpm where it was passing a lot of current but not turning fast enough to cool itself with its internal fan

Yes that video is a bit unrealistic so I recommend measuring your real situation.
I read maybe 105 to 120 amp alternator is common with some old ones at 65 amp and some replacements at 200 amp. Depending on the battery size I would look for no more than 30% to 50% of alternator rating. If the house battery is super low you could run the generator an hour before you roll.