Stirling Battery Charger

[XenoBlitz]

OK, I've fiddled around with the numbers a bit, and looked at design options... As far as I can tell it becomes a bit tricky when installing a large refrigerator and a couple of other extras to keep the dual batteries charged. For this my accomplice tells me to install a larger alternator.....

Now I like playing around with ideas, and this one came up, and the more I'm thinking about it, the more I am liking it, but of course it would always be nice to get some feedback from more experienced 4x4 drivers.

I can design and build a fairly nice stirling engine that can clamp on to Patricia's Exhaust, working off the heat... The approximate dimensions will be 200mmx200mmx80mm and the total weight will run around 5kg. (This means it cannot just hang onto the exhaust but will probably need to be mounted some way or another, but that is semantics.)

Because I'm trying to get this thing nice and efficient, it will be made from alluminum (mostly) and with the losses in the system, I'm calculating anything between 25 and 50 watts of power from her. (Thinking in terms of 12V that will be between 2 and 4.5 amps!)

Now before I start putting this into test and actually send the drawings to machine shop... Estimated cost R3000-R5000 (depends on quality which will influence those watts)... and with the added weight... will this actually be worth the effort? or is it simply better to go for the bigger alternator?

Love to here your thoughts about it...

[AndriesS]

Definitely engineer of some sort? Exhaust is a decent source of heat and free energy bit I really don't think it will be worth the effort -

As a matter of interest what will be your working gas? How will you isolate the vibration - small parts - tight tolerances and rough overlanding. Will the temp difference in close proximity of the exhaust and under the car on a hot day be enough to get the desired effect. Will the stirling engine rotate at the right speed for a given alternator/generator.

Might be a fun challenge but in the end you will most likely just upgrade the alternator.

[Alanvs]

A much easier way would be to just use a BC charger which can be purchased from Megaworld. This works like a normal battery charger but is DC to DC. I have one running in my truck and this chargers two Optimas in the back for the fridge. When I have my trailer connected it chargers a third battery with no problem. The nice thing about this charger is it has a small draw on the alternator.

[XenoBlitz]

Thank you for your replies. I'm not really the type of person to take on fun challenges just for fun... If the general opinion is that it just won't be worth it, then I suppose that is the case, and leave it there.

As in answers to your questions:
Yes, Mechanical Engineer
Working gas= air, non com pressed, that way sealing the unit won't be so much of a hassle, especially with the high temperature differences between ambient and operating.
Mounting will be slightly tricky, but with a few rubber connections, that can be sorted. (This still depends greatly on the mounting location. Fitting that size under the body without protruding I found this weekend requires moving of a few smaller components.)
Tolerances: Very easy: You specify them on the drawings and manufacture them accordingly.
Temperature difference: Hell yes. The proposed Stirling motor is based on VERY conservative numbers. By biggest concern is that we might actually get too much power from it and burn the electric generator. (Still considering going for a 6 amp generator instead. Just to be safe.) I've done a quick thermal analysis on the proposed design with some measurements taken from under the carriage while driving, and the results are astonishing.
Well, speed will probably be a little too high, so the voltage will have to be limited by electronics (This will be done simultaneously with the removing of the peaking in the stream.)

[Peter Connan]

Hello XenoBlitz

I am not sure how much power you are looking for, and how much you currently have available, but I beleive your alternator should be able to supply most if not all of your needs.

I operate a 60l Snomaster all the time, a 350w inverter for in-car entertainment for the kids (driving a laptop) and for charging camera batteries etc. When standing I also draw 3 strip lights and a long-life bulb, cellphone chargers etc. All this is powered from a single 105A/h deep-cycle battery in my engine bay, charged by a PBE charging system.

However I do beleive the particular dual battery system and the installation thereof is very important. My dad uses a National Luna system to drive similar appliances and battles cosiderably more than I do. I beleive the reason is that when my system is charging I have very nearly the same voltage at my second batery as at my alternator (this is even slightly higher than the charging voltage on the car's own battery), whereas my dad's system drops about 1 volt between alternator and second battery, due mostly to the Cole Hershee solenoid used in the National Luna system and a lot of the other systems on the market.

The result is that my battery charges at about 30Amps, for 360Watts, whereas my dad's battery seldom gets more than 10Amps, for 120Watts This problem gets even worse when the second battery is fairly well charged as the difference between charging voltage and battery voltage keeps on reducing and thus the charge rate keeps going down. Both these vehicles have standard alternators fitted.

On our recent 2-week trip to Bots my second battery never went below 90% capacity (12.35Volts), whereas my dad's seldom got above that. The PBE system is the only one I am aware off that does not have a voltage drop.

Regards
Peter