Background

The original lead acid battery (Interstate SRM24) had a capacity of only 84Ahr. Considering a lead acid battery can be damaged if drawn down more than 50% the usable capacity is only 41Ahr. Our normal usage is about 25Ahr/day so this battery is already marginal, less than 2 days. The new Nova Kool 'fridge usage should be about 43Ahr/day (4.4A running at 30-40% duty cycle). So, including the new 'fridge, our combined usage is about 68Ahr/day. This is less than one days use. The need for a more capable system was obvious.

Specifications

It was decided to convert to a lithium battery system. LiPo batteries are nice for RV use being smaller and lighter than an equivalent lead acid battery. They can also be charged faster. The new system will use the same charging methods as the original plus solar. A new converter/charger is needed as the original does not have a charging cycle for lithium batteries. Also, alternator charging requires a DC/DC charger instead of the simple battery issolator which doesn't support lithium batteries. Since no solar was previously installed, a complete system is needed.

An inverter will also be included. Ziggy has a very small inverter to run the TV on battery power. (This small inverter will be retained allowing the larger inverter to remain off while watching TV. The large inverter will have a larger parasitic draw than the small TV inverter.) A larger inverter is envisioned that will run all the outlets and microwave. Being able to run the microwave will be useful as it only runs for short periods. The air conditioner could be run, but any reasonable battery capacity wouldn't allow any useful period of operation. So for specifications running the microwave would be advantageous but the A/C isn't a consideration.

A general recommendation is to have the battery capacity to stay 2 or 3 days without recharging. A 200Ahr LiPo battery would give nearly 3 days use which is right in line with recommendations. We finally decided on a 300Ahr Epoch battery. This is only slightly larger physically than a 200Ahr battery and fits our space. A 300Ahr battery should give close to 4 1/2 days use. May be over speced but the buffer will be nice.

The recommendation for solar is 400-500w for a 200Ahr battery. As above a 200Ahr battery is a good size for our system, an added 100Ahrs being a useful buffer. Noting the ability to charge from shore power, generator, and alternator, as well as solar, the recommendations for a 200Ahr battery will fit our needs well. However, a 300Ahr battery could effectively us at least 750w of solar. The specification is to have at least 400w or solar with more, up to 750w, being advantagrous. The real plan is to put as many panels on the roof as will reasonable fit.

Other charging methods include shore, generator and alternator power. Shore power is based on a 120VAC 30A circuit which could supply 300A at 12V. Making use of this power for battery charging would require a charger to convert 300A. This is unreasonable for this sized system. Considering its use case, shore power will likely be connected for at least 8hr when available. To fully charge the 300Ahr battery would require 450w/hr (38A) for those 8hr. The charger should provide at least 38A (at 12v) to the battery. The generator is an Onan 2800 generating up to 2800w. This is similar to shore power, possibly supplying 200A at 12v. That would lead to a large charger. As the generator is a secondary source it should not drive the size of charger in itself. A size similar to that of shore power would be adequate, but a higher charge rate would be nice. A 120VAC charger of at least 38A is needed.

It should be noted, the system will pass enough 120VAC power to run the air conditioner. Figuring this at 1800w of 120VAC power, or 15A. However, it would be nice if the full 30A shore power could be passed to the coach's 120VAC circuits. (This becomes important when considering an inverter/charger with internal transfer switch.) This transfer switch, if present, needs to handle at least 15A (120ACV) and up to 30A could used effectively.

The stock Ford alternator has a 135A maximum output. This goes to charging the vehicle and chassis batteries as well as keeping the vehicle running. The limiting factor is overheating which is most likely when heavily loaded at idle, the lower RPM makes the fan less effective. So how much can we safely draw for the coach battery? Good question and not readily answered without measurements. But, from poking around the Internet a safe draw for an RV battery seems to be around 30-50% of maximum, provided prolonged charging while idling is avoided. That would suggest a coach battery maximum charging between 40A and 65A. Charging at 50A would require 6 hours driving to fully charge a 300Ahr battery, or some 1.5hr to replenish one days estimated draw (68Ahr). A specification here is to allow alternator charging at up to 50A.

The inverter needs to at least run the small microwave. Microwaves draw between 600w and 1000w. The conservative specification will specify an inverter of at least 1500w considering the microwave running with other smaller loads.

Summary

In summary we're aiming for a system based around a 300Ahr lithium battery. The solar contribution to be at least 400w but larger if roof space allows. Shore and generator power should each be able to supply at least 38A (12VDC) for charging, with a higher limit being better. Alternator charging to be included at up to 50A. It will include an inverter providing at least 1500w to the coach's 120VAC circuits. Finally, if an inverter/charger transfer switch is included it needs to pass at least 15A (120VAC), with a higher value, up to 30A, being prefered.