How Is the Battery System Managed?
If you have only used lead acid batteries, then the words Battery Management System (BMS) might be new to you. A BMS is a necessity if you plan to use lithium batteries. Lithium batteries are not the dumb bricks that lead acid batteries are and require active management to keep them healthy and operating safely.
If you buy a 12-volt lithium ion Group 27 or Group 31 replacement battery, a BMS is located inside the battery and operates without your knowledge or involvement. Any other lithium battery construction will require a BMS in the battery design and construction.
Every BMS does at least three things; stops discharge at low voltage cutoff, stops charge at high voltage cutoff, and balances cell voltages. Most do more, including preventing excessive current draw. The first and second functions should be self explanatory, but cell balancing is something new.
As cells discharge, the cell voltages drop differently. As cells charge, they do not all take the same charge and return to the same voltages. Over time and charge/discharge cycles, the differences become large enough to affect battery performance and are not self-correcting.
In order for batteries to maintain their high capacity, the cells all need to be kept in the same voltage range artificially to maintain equivalent power contributions to the battery. To do this, a simple BMS will bleed off the voltage of the higher cells. A more complex BMS will actually use a higher cell to charge a lower cell.
The BMS I selected has many additional control parameters. It can stop either the AC or DC battery charger. It can shut off the inverter. It can engage the solar charging. And it monitors all DC power used in the camper.
How Am I Going To Charge A Battery Bank This Big?
I need two significant, reliable and redundant means to charge this large battery bank in addition to whatever I can get from solar. Solar is just not powerful enough or reliable enough to meet my robust goals.
The most cost efficient and quickest charge will be from AC shore power. The camper has a 30-amp/120-volt shore power cord. If I were able to charge with everything the shore connection can provide, that would be 120-volts x 30-amps = 3.6kW.
With 3.6kW input and a 95-percent charger efficiency, I can probably expect to get 3.6kW x 95-percent = 3.42kW delivered to the battery.
Assuming an average charging voltage of 28.8-volts (normal for a 24-volt system), I can expect to power a charger capable of 3.42kW/28.8-volts = 118.75-amps.
I very purposefully chose a large inverter with an integral charger that could handle lithium profiles. The Samlex EVO 4024 inverter has a 110-amp built in charger. At maximum charge rate, this charger can recover a fully discharged battery in; 13.44Wh (battery capacity) / 3.168kW (charger capacity) = 4.24-hours.
My second charging source could be an onboard AC generator. A generator running four about four-hours would also charge the batteries, but that’s a lot of noise for a long time. I just don’t want that.
I never really wanted to give up the weight or space for an onboard generator anyway. It is also not a fully redundant system since it shares a common charger with the AC shore system.
Instead, I decided to use a DC-charging system by adding a second, dedicated alternator onto the truck’s engine. F-series Ford trucks offer a second alternator as an option. I was able to buy an entire drive and mounting set from a used parts dealer and installed it.
The second alternator belt system is completely separate from the engine equipment belt system. I bought a new stock alternator (120-amp) and converted the internal 14.4-volt constant voltage regulator to an external constant, but adjustable voltage regulator set to 29.4-volts.
This is not as optimal as a special battery-charging regulator used on sailboats, but is one-fifth the cost and works pretty well. It provides 60-amps when the battery is nearly full and up to 150-amps when the battery is nearly empty.
I have a 4-gauge cable with a 175-amp connector beside my vehicle’s light harness. This connects to the camper and allows me to charge at the same time that I am moving to a new location. I tend to do this every three to four days.
This is not the most economical method, but it is quite painless and simple. This system also has a manual shutoff, alternator temperature cutout, and is controlled by the BMS.