Battery sizing

[Charliesmom]

We have a battery bank of 360 AH LiFePO4 in our 2019 Transit SMB. We are able to charge the batteries with the engine alternator (heavy duty option) using a DC to DC charger that provides 50 amps while driving, 230W solar panels on the roof with an additional 120W portable suitcase, a small portable generator (Yamaha 2000), if needed, and shore power when available. The key to power management in these rigs is to understand your electrical requirements, have a good BMS onboard to monitor your consumption (Victron 712 BMS in our case) and have multiple options available to charge the batteries.
Good luck with your planning. This is the fun stuff.

[Twoxentrix]

^^^^ You can also draw down the Firefly Carbon Foam to 80%^^^^^
Lifecycles are approximately 3 times more than AGM, but 1/2 of Lithium.

https://coastalclimatecontrol.com/in...g-reveals.html

[Wowbagger]

With regards to charging Lithium off the alternator: my situation is different from 90%+ of people in that my lithium system is 24V rather than 12V, so I *have* to have a DC to DC converter to charge from the alternator. The rest of my comments are, therefor, not from direct experience, but rather from being an electrical engineer with some background in the subject.
The nominal float voltage for lead acid is around 13.8V - a bit higher for AGM, but close enough. Most vehicle charging systems will run right around 13.8V, to get the best life out of the battery.
The nominal float for LiMnFeO4 is about 14.4V. Thus, on most vehicle charging systems, if you just parallel the lithium with the chassis battery, it will not get fully charged from the alternator. If you adjust the voltage regulator on the alternator (not easy!) you could get the 14.4V you need, but then you will slowly boil the vehicle battery dry. This is why in most systems a DC/DC battery charger is needed - to boost the 13.8V to the charging voltage for the lithium batteries.
The second issue is that lithium batteries have a very low resistance - they are very "thirsty" and will suck down a tremendous number of amps while charging if they are low. A 100Ah lithium battery can consume between 100A and 300A of current during charge - most of the integrated BMS batteries limit themselves to 1C, so 100A for a 100Ah battery, but then that's still 100A for an hour. Many vehicles have a 120A alternator, and need about 60A for the vehicle systems, so if you have that lithium battery saying "FEED ME!" you just smoked your alternator. Moreover, even if it doesn't smoke right off, you will be running max for hour(s) - and many alternators aren't rated for that. Again, that becomes a MBSRE - Magic Blue Smoke Releasing Event.
The idea is that a DC/DC converter can tame this - keep the current draw to what the alternator can supply, keep the voltages correct, etc.


Back to my direct experience: My system has a DC/DC converter that can draw upwards of 110A from the 12V system, and supply up to 50A to the house batteries. My Sportsmobile has a 220A alternator, so it has enough heft to take the draw. I have the float voltage set to 27.0V - a little under full charge, but close enough. This helps prolong the battery life while still providing plenty of stored energy.

[larrie]

The Fireflys look like a possible solution.

[boywonder]

Originally Posted by Wowbagger

The idea is that a DC/DC converter can tame this - keep the current draw to what the alternator can supply, keep the voltages correct, etc.

How does a DC/DC converter limit current to what the alternator can supply?

[Scalf77]

The DCtoDC charger replaces the load of the house battery. If you have a 50 amp DCtoDC charger, the load would be 50 plus conversion cost. Where as if directly tied to the battery it may want more than that 50 plus number. Some can also monitor the output voltage of the alternator and throttle it back if the demands on the alternator is two high.

-greg

[Otter]

Originally Posted by larrie

Now that the subject of lithium has come up, I have see and read mixed comments about charging them off a vehicles alternator while driving. Some people say yes others no. Anyone have a definitive answer? Would like to go lithium and need to resolve the charging issue first.

Thanks

Hi Larrie. You might want to come up and see my new lithium install and/or email/call me with any questions. It’s a 200ah Battle Born-based system with solar. Charges off the standard alternator with a 50 amp DC to DC Charger. So far, even short trips are topping it off well; the rooftop solar isn’t hooked up yet.

Sure, the batteries are expensive, but BB gave me a discount on the order when I called, free shipping, 10 year warranty. And lithium gives me 160 ah to play with with no worries.





Take a look at my recent post #205 on this here:

https://www.sportsmobileforum.com/fo...-17377-21.html

.

[Besags]

If you are considering the Firefly batteries make sure you check the G31 dimensions. They are slightly taller, a little less deep and two of them become slightly wider than a 4D battery. I have a 07’ E-350 Sportsmobile with the RB 50 layout. The house batteries are located underneath the floor just aft of the rear axle in a tray hanging from the frame. I was able to remove and re-weld the tray side stops to accommodate the extra width of the two combined Firefly’s. The height was a tight fit, but it worked out fine. i also installed a Kisea DMT-50 DC to DC battery charger and a Balmar SG200 battery monitor with the bluetooth option. The old solar controller was removed and the solar panel hooked up to the Kisea unit. So far the setup has worked great. I especially like being able to monitor what’s happening to the house batteries with the Balmar app on my smartphone.

[Orv]

It's worth checking what your alternator puts out. Mine reliably charges at over 14V, which should work fine for lithium batteries. However, on other cars I've seen Bosch alternators that only put out 13.8V. Also, most alternators are temperature compensated and will reduce charging voltage at higher temperatures.


Another potential issue is what kind of weather you camp in -- many Li-Ion batteries disallow charging below a certain temperature.

[pjtpj]

There is some great info on this thread. There are just a few key details missing, so I'll try and fill those in:

When computing the usage of an appliance like a fridge, you have to consider the "duty cycle" - the amount of time that the big loads like the compressor are actually running. This is a pretty good writeup on a Norcold NR751 that is the size typically installed in vans like ours: https://www.buildagreenrv.com/design...ectricity-use/. Considering duty cycle, the the NR751 average use is 2ah @12V.

On a 200ah AGM battery, you can use all 200ah, but if you repeatedly use all 100% of the capacity (Depth of Discharge/DOD) you reduce the useful life of the battery. Top quality battery vendors publish curves that show how battery life varies by DOD. Here are the stats for our AGM batteries: 700 charge/discharge cycles @ 100% DOD, 1,600 cycles @ 50% DOD, 3,000 cycles @ 25% DOD.

One of the big improvements with lithium batteries is you can use 100% of capacity with a minimal battery life reduction. So, a lithium battery might allow 3,000 cycles @ 100% DOD.

For either AGM or Lithium, if you charge or discharge the battery at currents that are too high, bad things can happen. The battery's pressure relief valve vent might open discharging fumes and causing a rapid capacity loss. You might blow a big, expensive and hard to replace fuse. You might melt something or start a fire. You could cause your chassis to malfunction if you pull the chassis voltage too low.

Lithium batteries can be damaged by charging below 0F (exact temperature varies) or overcharging. As mentioned above, currents that are too high can also cause damage. A typical BMS can't really solve these issue, but it can disconnect the battery if it detects that the battery is being operated in a way that could cause damage.

Both AGM and lithium batteries have reduced capacity at low temperatures. High temperatures can cause other types of problems.

To get a complete charge, both AGM and lithum batteries need to be charged with a "charge profile" that matches the battery design. If you use a non-ideal profile to charge your battery the results vary from suboptimal battery capacity to battery damage. Lithium batteries need to be balanced to get a complete charge. The balancing function can be built into the BMS or it can be a separate device.