Setup / Requirements
The electrical system is made up of three+ components:
- Generator (charger): Wrico 10kW 1800RPM liquid cooled genset in a stealth doghouse
- Batteries: 10 DEKA Gel Group 8D batteries
- Inverters: Two Xantrex SW4024 (only one shown in current album photo)
- Solar: Twelve 85W Panels (1kW) + Outback FlexMax 80
Most bus/RV setups use the generator for when they need full power
and can't hook up to land ("shore") power. This won't work for me
since I won't have shore hookups where I'm headed.
I need to have full electrical power available at all times - I don't
go into a "low-power" mode when travelling because I'm always
travelling. So the battery pack will be massive enough to power
an A/C unit (RV culture presumes this to be impossible) and I'll
have inverters to match. Because of this I only need the generator
for charging the batteries.
In fact, this generated quite a bit of discussion
on the BusNut Forum, so I was quite happy to prove them wrong.
Here are my first (very rough) guesses at power usage, showing the
calculations I made to arrive at my battery/inverter/genset sizes.
power h/day Wh/day
----- ----- ------
Computer 100W? 24h 2400Wh
19" LCD 50W? 8h? 400Wh
Powerbook 25W 6h 150Wh
Touchpads 20W? 10h 200Wh
Lights 200W? 15h 3000Wh
Stereo? 200W? 2h? 400Wh
A/C 1500W 6h?? 9000Wh
Water pump 40W 1h 40Wh
Furnace Fans 90W 2h? 180Wh
I'm going to guess that I'm forgetting some power needs and I'll leave some
space for more A/C, so I'll round daily consumption up to around 20kWh
per day (in hot climates).
I'm going to generally round up in my calculations, because running
out of power would be bad.
Also, figure efficiency losses of inverters and chargers and wires by
adding 25%, I'll set it at 24kWh/day. This is a lot. One thing I
can do is go to a lower power mode only when in very hot climates,
since A/C is about 50% of my usage (and even then it's with only one A/C
running - I hope to spend minimal time in areas where I need both, I'll
have to run my generator more hours/day then). If I can afford it, I
might switch to DC powered A/C, they are far more efficient (and more
Our inverter needs to be over 2kW if it's going to handle everything
running without shutting down, especially consider the A/C motor spikes.
Again, I'd like to oversize here,
because having a shutdown when I was gone could be catastrophic. If
I want to use the other A/C then I'll need another inverter, but I
can get this later. Eventually I'd like 6kW of max power for a full
range. Two 3kW inverters could work fine, especially if I get load-balancing
caps on the A/C units to avoid A/C motor spikes.
Note: These calculations are no longer true - I use less power,
though still more than most mobile setups, but it's not 24kWh,
and I don't recommend 50% depth of discharge, unless you have Gels
I'm told that a "day of autonomy" for 24kWh of batteries is about
33kWh of charge. That's somewhat close to my calcs, where I'll discharge
batteries to about 50% before recharge, and I'd like to only have
to recharge twice a day. At 24kWh/day use, I need to charge 12kWh
each time, if that's 50% of my batteries, then I need about 24kWh
of batteries. If I run a 24V system, that's about 1000Ah or more
of charge. I've been told that I need even more than that, batteries
being what they are...
This is a heck of a lot, so I might cut back on this. For maximum
battery life I am told one should aim for 2-3 days of autonomy, but I
don't care as much about whether the batteries last 5 or 20 years
versus the total cost, weight and space, so skimping on "days of
autonomy" makes sense.
I'll be doing half a discharge in 12 hours. This means my average
discharge rate is a full 24 hours (about 42A). A healthy discharge
rate is 20 hours (Ah/20) so I should be fine - though I'll have spikes
of about twice that current at times, giving me a temporary discharge
rate of 12 hours. I'm pretty sure this won't melt the batteries, but
it will shorten their life somewhat. A worthy tradeoff for me.
In the end I bought 29.4kWh of batteries. It was about all I could
fit in the bay and my wallet - initial tests imply that this will be
enough juice for what I want to do.
Generator / Charger
As calc'd above, I want to charge about 12kWh twice a day, I'd
prefer to only have to run the genset each time for about 2-4 hours.
(Note: This ignores the inefficiencies of charging, but more importantly
it ignores the fact that batteries will not take whatever amount of
current you want to dump into them, and they often need to float for
some time to stay healthy. So this means longer charge times than
calculated - though as mentioned I'm not using as many kWh as originally
calculated anyways. Generally the bank only takes max charge at the
beginning of bulk and then it curves down over time)
If I charge 12kWh in 2 hours, then I need a 6kW charge. That's
about 250A of charge, and I'd probably need a 8-10kW genset to
maintain that. Inefficiencies everywhere. Considering the size
of the bay that I want to put the genset in, I might go with 8kW
and just allow for the possibility of having to run the genset
for longer on hot days. When I don't need A/C, I'll probably be
able to get away with running for an hour twice a day or less.
The inverters generally come with 85-100A of charge, so I'll
probably need to buy one or more chargers to parallel with that.
A safe charge rate is Ah/5 (full charge in 5 hours) which is
4.8kWh/h, or 200A. I probably shouldn't push over 10kW from
a charging perspective, but I think 6kW should be close enough,
more investigation is required.
I want to have the genset start and stop automatically. I'm
told that the autostart feature might kick in everytime I
have a power surge (though this is lessened by having a large
battery bank). Some options:
None of these options were great, but now there is a solution!
Bogart Engineering has added a relay control to their pentametric
meter which can calculate the state of charge of a battery bank
(based on actual Ah draw). Problem solved.
- Use autostart and hope it works
- Use Ah counter to determine restart time, double-check
actual battery levels every few months?
- Have computer watch voltage level and ignore surges,
control charging (ideal, but much work)
- Start generator every day at set times, shutoff when charged
(Added value of having time-of-day control over generator noise)
- Combine #4 with autostart set very low for safety backup.
All material © 2006, David Ljung Madison