Battery Monitoring with Thornwave

Last summer I added a 100 amp-hour lithium (LiFePo4) battery to the campervan, and last fall I switched out the OEM flooded lead acid batteries for a pair of absorbed glass mat (AGM) batteries.

In the current setup:

  • Either battery bank can be charged by any or all of the campervan alternator, the 200w rooftop solar, by plugging the camper into wall (shore) power, or by running the Onan generator hanging under the campervan behind the rear axle.
  • The alternator can charge at a rate of at least 40 amps, and the charger/converter can charge at a 50 amp rate when plugged in or on generator. We use somewhere between 25 and 40 amp-hours overnight, so either driving or running the generator for an hour recovers most of what we use.
  • The 12 volt lights and appliances can be run from either battery bank. Switching the load between banks is manual.
  • The 120 volt microwave, air conditioner, and outlets can only be used if the campervan is either plugged in or the generator is running.

The combination works OK under most conditions. In moderate weather, the AGM’s will run the fridge and lights overnight with some capacity to spare. The lithium battery will easily last overnight even with the furnace running. Because we didn’t spend more than one night without charging the batteries by either driving or plugging in to wall power, I don’t know if we could run two nights in a row without driving, plugging in or topping of with solar.

An unknown has been the exact state of the AGM batteries as they charge and discharge. I installed a Xantrax LinkLITE battery monitor and shunt when the campervan was new, but until this winter I didn’t realize that because of the way the AGM batteries are grounded the Xantrax shunt was not seeing all the current going in and out of the AGM batteries. It would miss any current coming from the engine alternator an unknown fraction of the current coming from the charger/converter and solar, and hence was not accurate most of the time.

I attempted to force all ground paths through the shunt by adding new and removing old chassis grounds. Turns out that the Onan generator frame is a low resistance path to ground that I can’t separate from the chassis, so a ground side shunt isn’t going to work. In other words, the Xantrax has to be configured to use a shunt on the ground side. I can’t shunt the ground side.

Instead I’ve purchased another Thornwave battery monitor and installed it on the +12v side of the AGM batteries. Because I can force the +12v positive side of the AGMs through a singe wire, the monitor should accurately measure current to and from the AGM’s and I should be able to determine the battery charge state and remaining capacity.

The Xantrax is now for sale for (half of what I paid). It’s a good monitor, it just doesn’t work when you have lots of paths to ground.

The Thornwave is Bluetooth-only. You must be within Bluetooth range and have either an Android or iPhone apps to view battery state. Once within rage, you have access to monitoring and configuration. The Thornwave may be connected to a relay and configured to control the relay based on battery voltage, current, state of charge, temperature and/or time.

I’m currently experimenting with using the Thornwave to control a solid-state relay that disconnects the Redarc battery charge when any of the conditions are met:

  • AGM battery voltage below a threshold (o provide finer control over the Redarc charger). My theory is that the thresholds can be set to keep the lithium from being charged until the AGM’s are fully charged, or vis versa.
  • Ambient temperature is below freezing (to prevent lithium battery from being damaged by charging at too cold of a temperature.
  • Charge current is over a threshold – just in case something goes wrong.
  • When I want to connect or disconnect without having to use the mechanical switch, I.E. when I’m driving.

The ‘as built’ campervan electrical is described in detail on the Campervan Electrical page.

2 Comments

  1. Do you think your Xantrax would work well with a fiberglass trailer such as a Casita with limited ground paths?

  2. John – If you can insert a shunt in-line in your battery ground cable between the battery and the frame, you should be OK.

    My problem is that the Onan generator is grounded to both the truck chassis and the coach batteries, so there is no spot for a shunt.

    [It looks like the Casita battery is in a compartment, so odds are that the shunt could be mounted in the compartment.]

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