Those of you who are planning to upgrade your late model National RV motorhome batteries from lead acid to lithium ion, it is not a straight forward process. I don’t have a wide range of experience with motorhome battery management systems, but after upgrading my 2007 National RV Seabreeze to LiFePO4 house batteries while leaving the flooded lead acid chassis battery, I know quite a bit about that battery management system and I’ll share that here.
The National RV system (manufactured by RV Custom Products) uses an interconnect solenoid to connect the house and chassis batteries under the following circumstances:
• When on shore or generator power and the house batteries are fully charged, the house and chassis batteries are connected by the solenoid so that the chassis battery can be charged by the inverter/charger (or converter).
• When driving and the chassis battery is fully charged, the house and chassis batteries are connected by the solenoid so that the house batteries can be charged by the alternator.
• When off grid and the chassis battery is too depleted to crank the engine or generator, the house and chassis batteries can be connected by the solenoid by pressing the “emergency start” button on the dashboard. The RV Custom Products controller activates the Solenoid for one minute. Other MHs that I have owned require you to hold the switch while starting.
I only replaced my house batteries with lithium and left the chassis batteries as lead acid. Since lead acid and lithium ion batteries have vastly different charging requirements, using the solenoid to facilitate charging the house batteries from the alternator and the chassis batteries from the inverter/charger will not work; however, the solenoid is required for the emergency start and we don’t want to lose the that function. If one was to upgrade both the house and chassis batteries to lithium, the solenoid could still be used for charging as long as the alternator is protected by a DC-DC charger (see below).
With the inverter/charger configured for lithium (AGM, actually), it cannot be used to charge the flooded lead acid chassis battery. The solution I used for that is to use an inexpensive battery charger powered by the inverter AC input circuit. (Any AC circuit would work but the inverter circuit is available in the Seabreeze compartment that houses the battery control circuitry.) That one was easy.
Lithium batteries will take all of the charging amperage that you can throw at them. Lead acid batteries can only absorb so much. If you hook a lithium battery directly to the alternator driven chassis battery charging system, it will quickly burn up the alternator. In order to charge the lithium batteries from an alternator, a DC-DC charger is used to control the amperage and charging profile. The input current to the DC-DC charger comes from the chassis battery (which is connected to the alternator) and the output goes to the house batteries. There is an input signal to the DC-DC charger that tells it when to charge. That input is connected to the ignition switch so that it only charges when the ignition is on. (This is not optimal in that if the ignition is turned on but the engine is not started, the charger will draw from the chassis battery without the alternator replenishing it, so you don’t want to have the ignition on without the engine running for very long or it will drain the chassis battery. Some people opt for a dashboard switch to control when the charger is active.) I took the input power for the DC-DC charger as well as the input signal mentioned above from the relay that is on when the ignition is on (J5 on the circuit board) and I fed the charger output to the house batteries through J7 (that the inverter/charger is connected to).
It would be easy to implement the emergency start functionality that requires the dashboard switch to be held while cranking the engine; however, that would involve the dashboard switch and the wiring between the switch and solenoid to carry the current that the solenoid requires to operate. Rather than go that route, I opted to use a small microcontroller to operate a relay which energizes the solenoid. The dashboard switch connects one of the microcontroller inputs to ground drawing virtually no current. When the microcontroller sees the input go to ground, it energizes the relay which in turn energizes the solenoid. It leaves the relay and solenoid energized for one minute, replicating the original behavior. You have to disconnect the activation wire from the solenoid (the yellow wire that comes from the circuit board to the solenoid). It gets replaced by the wire from the relay that is controlled by the microcontroller.
That is what I did to upgrade my Seabreeze. If one wanted to do a trivial implementation where the house batteries are only charged from the inverter/charger and the chassis batteries are only charged from the alternator and there is no emergency start function, just drop the new batteries in, and disconnect the activation wire (the yellow wire) from the solenoid. In any event, you will have to change the settings on the inverter/charger to lithium (or AGM if there is no lithium setting).
I used two of these batteries in parallel for 240AH total capacity. https://smile.amazon.com/Lithium-Pho...0871339&sr=8-5
I used this DC-DC charger. https://smile.amazon.com/gp/product/...?ie=UTF8&psc=1
I used this AC-DC battery charger: https://smile.amazon.com/Schumacher-...s%2C515&sr=8-7
Here is a link to the RV Custom Products page containing the schematics for the battery management system. The Word document linked to on that page is very helpful. https://rvcustomproducts.com/130325
I used this microcontroller: https://smile.amazon.com/gp/product/...?ie=UTF8&psc=1
Hooked to this relay: https://smile.amazon.com/gp/product/...?ie=UTF8&psc=1
Except for the emergency start function, this is totally a DIY friendly project as long as you know a little about 12 volt circuitry (auto electrics). If any of this intimidates you, consider getting professional help. You are welcome to pick my brain here… just post questions in a comment.
As for the emergency start function, if you know anything about Arduino microcontrollers, this is a very easy application. If not, it is easy and fun to learn. (Just Google “Arduino”.)
Hope this is helpful to someone!