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]

Thanks for all the helpful advice. I'll be sure to use adequate wire, proper fuses or circuit breakers, and make sure the charger is ventilated and protected.

[pianotuna]

Teleman wrote:
If I can mount it in a plastic box safely I can keep the wires really short, 12-18", as the starter and house batteries are located very close to each other under the hood. Renogy recommendso 6ga from the starter battery up to 8' and a 60a fuse, and 8ga and a 50a fuse to the house battery.

If there is room--use a plastic battery box to hold the renogy. Make sure there is plenty of ventilation. dc to DC are not wonderfully efficient so the unit may need to dissipate over 250 watts of heat.

One minor concern. Is the starter battery flooded? If so the renogy should not be in the same compartment (corrosion will eat the renogy up). If the starter is AGM, then that's ok for the renogy to be in the same compartment.

I use auto reset circuit breakers rather than fuses.

I have dual 50amp charging paths (an attempt to improve charging before dc to DC devices were available). I've seen 70 amps--back in the old days when I had flooded house batteries (975 amp hours). I would use my inverter while running down the road and turn on the water heater. I used a duty cycle of 20 minutes on and 40 minutes off. I have manual control over the isolation solenoids.

[otrfun]

Teleman wrote:

otrfun wrote:

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!

If I can mount it in a plastic box safely I can keep the wires really short, 12-18", as the starter and house batteries are located very close to each other under the hood. Renogy recommendso 6ga from the starter battery up to 8' and a 60a fuse, and 8ga and a 50a fuse to the house battery.

The 40a Renogy's case gets very warm plus the two fans will activate whenever it's producing its rated 40a of charge current for any length of time. If you install it inside a plastic box I'd make sure it's well ventilated.

A 2ft long 6 gauge cable run will net you an extremely low .6% voltage drop at 45a. IMO, anything less than 2% is excellent. If you keep your voltage drop on the input at <2% you should easily see less than a 45a load on the input (with 40a on the output).

As for your output, 8 gauge will net you less than a 2% voltage drop at 40a as long as it's less than 5ft.

We followed Renogy's fusing recommendation (60a input, 50a output).

Good luck!

[pianotuna]

The purpose of the bypass switch.

Scenario.

You are not on shore power. There is no sun for solar. You want to charge the house batteries. The dc to DC renogy fails.

So use the bypass switch to do some charging.

Other advice:

Once you have the dc to DC installed, use a DC clamp on ammeter to check the current. If it is more than 50 amps, pull the "jumper" out of the renogy to limit it to 20 amps.

Input of the dc to DC can be 50% more than the output. So 40 amps out may equal 60 amps in. My opinion is that is far too much for a 124 amp alternator--unless the duty rating is 100% (they rarely are).

[Teleman]

otrfun wrote:

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!

If I can mount it in a plastic box safely I can keep the wires really short, 12-18", as the starter and house batteries are located very close to each other under the hood. Renogy recommendso 6ga from the starter battery up to 8' and a 60a fuse, and 8ga and a 50a fuse to the house battery.

[StirCrazy]

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

get a remote thermoniter and put it in the battery location and see how hot it actualy gets there, stop and go trafic in the summer will probably be the hottest. if it is in a location that stays cool then your fine, but most wont. the house battery isnt a big deal to move, its just runing wires from the under hood location to the new location. with the LFP you dont have to worry about spills or off gasing so you can put them anywhere. your DC to DC charger you want as close to the batteries as possible. and even in the situation where you cant move them it might be fine with a simple heat shield as in reality are you using the house batteries while your travleing?

Steve

[otrfun]

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!

[3_tons]

theoldwizard1 wrote:

pianotuna wrote:
Some folks say it is best to add a dc to DC converter to prevent the alternator from overloading.

Concur !

I think the alternator overload is an Internet rumor, but a DC-DC charger, properly configured, will do the best job charging your house batteries.

Agreed, but since he already has a Victron ‘smart’ shunt, I’d go with a Victron dc-to-dc charger since it too is a Victron format Smart phone capable…

3 tons

[Teleman]

pianotuna wrote:
#4 wire is overkill. #8 would be sufficient.

I would put the dc to dc device between the starter battery and the house bank.

That will make for a short run.

Put the dc to DC in a plastic box with no lid to shield it from splashes.

I would add a switched bypass for the dc to dc device.

I was afraid it would overheat in an enclosed box but if not that makes instalation easy. The starter battery and house batteries are very close to each other. Whats the purpose of the switched bypass?

[otrfun]

Gdetrailer wrote:

otrfun wrote:
The gist of my comment was to acknowledge a fuses lack of precision. Nothing more, nothing less. *If* a given 60a fuse would precisely open at 61a, then, yes, it could be effectively used for current limiting purposes (i.e., protect an alternator from excess battery charge current). Unfortunately, this is not the case. Given batch of 60a fuses of the same type and rating, they may open at 75a, 95a, or even 120a. These are totally unacceptable tolerances in respect to effectively protecting an alternator from excess current while directly charging a battery.

The "gist" of my comment towards your comments is even IF you were to use a fuse that was say 60A and blew open at 61A that would be a total misapplication if you were depending on a fuse to do this.

That would be pretty dumb to blow and replace a fuse each time you hit say 61A when there ARE devices out there that do that job far better automatically without the need to replace fuses or turn breakers back on.

Fuses and breakers are designed to protect the wiring from becoming the fuse and causing a fire, nothing more and nothing less than that.

Fuses are not "regulators" they are for catastrophic events like a short circuit.

If you want or need current limiting then you need regulator devices which are designed specifically to limit current draw (they do so by reducing the output voltage which limits the max output current).

Switching power supplies like the DC to DC charging units being discussed in this thread typically will have output current and/or voltage limiting already prebaked in.

If you are concerned about how much load the DC to DC power supply charger can draw from your alternator source then select a DC to DC power supply charger with a lower amperage output.

In other words, if 60A is the absolute max you want to draw from the alternator, then select a DC to DC power supply charger that has no more than 40A output capability or even less.

As always, include fuses from your power sources. Select fuse/breaker size to the max allowable amperage of your wire, place it as close to the battery(ies) as possible. Should have a fuse on the vehicle side and a fuse on the lithium battery side and the DC to DC charger goes in between both fuses.

Wow. Did you not see the "*" around the word if? It was a hypothetical to describe the type of precision necessary to safely and effectively protect an alternator while charging a battery directly. No such metallic fuse with such precision exists, so it's clearly a hypothetical.

To be clear: I am NOT, I repeat, NOT, proposing the use of a fuse to limit alternator current to the battery. Why? Because a typical metallic fuse doesn't have the precision or close tolerance to do so safely and effectively.

If you read my previous post I clearly support the use of a dc to dc charger. A 40a dc to dc charger is installed in our truck camper. We run our 11k a/c unit with our 200ah lifepo4 all the time when we're on the road. Need the dc to dc charger to charge our lifepo4 back-up in-time for next break. This dc to dc charger gets one heckuva work out everytime we're on the road during the summer. Works great.

[pianotuna]

#4 wire is overkill. #8 would be sufficient.

I would put the dc to dc device between the starter battery and the house bank.

That will make for a short run.

Put the dc to DC in a plastic box with no lid to shield it from splashes.

I would add a switched bypass for the dc to dc device.

[Gdetrailer]

otrfun wrote:
The gist of my comment was to acknowledge a fuses lack of precision. Nothing more, nothing less. *If* a given 60a fuse would precisely open at 61a, then, yes, it could be effectively used for current limiting purposes (i.e., protect an alternator from excess battery charge current). Unfortunately, this is not the case. Given batch of 60a fuses of the same type and rating, they may open at 75a, 95a, or even 120a. These are totally unacceptable tolerances in respect to effectively protecting an alternator from excess current while directly charging a battery.

The "gist" of my comment towards your comments is even IF you were to use a fuse that was say 60A and blew open at 61A that would be a total misapplication if you were depending on a fuse to do this.

That would be pretty dumb to blow and replace a fuse each time you hit say 61A when there ARE devices out there that do that job far better automatically without the need to replace fuses or turn breakers back on.

Fuses and breakers are designed to protect the wiring from becoming the fuse and causing a fire, nothing more and nothing less than that.

Fuses are not "regulators" they are for catastrophic events like a short circuit.

If you want or need current limiting then you need regulator devices which are designed specifically to limit current draw (they do so by reducing the output voltage which limits the max output current).

Switching power supplies like the DC to DC charging units being discussed in this thread typically will have output current and/or voltage limiting already prebaked in.

If you are concerned about how much load the DC to DC power supply charger can draw from your alternator source then select a DC to DC power supply charger with a lower amperage output.

In other words, if 60A is the absolute max you want to draw from the alternator, then select a DC to DC power supply charger that has no more than 40A output capability or even less.

As always, include fuses from your power sources. Select fuse/breaker size to the max allowable amperage of your wire, place it as close to the battery(ies) as possible. Should have a fuse on the vehicle side and a fuse on the lithium battery side and the DC to DC charger goes in between both fuses.

[Teleman]

theoldwizard1 wrote:

pianotuna wrote:
Some folks say it is best to add a dc to DC converter to prevent the alternator from overloading.

Concur !

I think the alternator overload is an Internet rumor, but a DC-DC charger, properly configured, will do the best job charging your house batteries.

I just ordered a 40 amp Renogy charger. Now I have to figure out where to mount it and get sufficient 4 ga wire. Stuff ain't cheap...

[theoldwizard1]

pianotuna wrote:
Some folks say it is best to add a dc to DC converter to prevent the alternator from overloading.

Concur !

I think the alternator overload is an Internet rumor, but a DC-DC charger, properly configured, will do the best job charging your house batteries.

[otrfun]

Gdetrailer wrote:

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.

The gist of my comment was to acknowledge a fuses lack of precision. Nothing more, nothing less. *If* a given 60a fuse would precisely open at 61a, then, yes, it could be effectively used for current limiting purposes (i.e., protect an alternator from excess battery charge current). Unfortunately, this is not the case. Given batch of 60a fuses of the same type and rating, they may open at 75a, 95a, or even 120a. These are totally unacceptable tolerances in respect to effectively protecting an alternator from excess current while directly charging a battery.