Alternator delete with lithium and lead acid battery

[SVOboy]

Brucepick has been working on an interesting alternator delete for a while now. He wanted to eliminate the load on the alternator, but didn’t like the idea of just replacing the starting battery with a deep cycle lead acid battery. The downside of doing this is that as the battery discharges the voltage sags lower [...]

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[JRMichler]

I had similar thoughts recently when I realized that, with a certain amount of effort, I could get my summer average to 40 MPG.

My idea is a two position alternator switch. Position 1 would be "normal". Position 2 would shut off the alternator. The truck would continue to run off the standard battery. A second deep cycle battery would charge the standard battery through a DC-DC converter with about 14 volt output. The second battery would be charged from a flexible solar panel mounted on my topper and charge controller. I would want the deep cycle battery to be sized for four hours at 20 amps into the standard battery. The solar panel should fully recharge the deep cycle battery in 8 to 12 hours.

I don't think I'll ever do it, but it's a great thought experiment.

[tortoise]

So if I want to just drop in a deep cycle battery and remove the drive belt on my Metro, what's a reasonable guess for my safe range (miles per amp hour)before recharging? I'm guessing cranking power isn't an issue. Yes, I know there are some imponderables, I'm looking for a ballpark number. Also, what reasonably priced, readily available batteries have a decent amp-hour/pound ratio?

[Daox]

Most guys just replace their normal starting battery with a lead acid deep cycle battery. My group 24 (probably ~70Ah) battery in my Paseo would easily get me back and forth to work a little over 40 miles when I had that commute. It was fine even with my fan blasting in summer, and wipers/headlights going in rain.

[paulgato]

Originally Posted by JRMichler

I had similar thoughts recently when I realized that, with a certain amount of effort, I could get my summer average to 40 MPG.

My idea is a two position alternator switch. Position 1 would be "normal". Position 2 would shut off the alternator. The truck would continue to run off the standard battery. A second deep cycle battery would charge the standard battery through a DC-DC converter with about 14 volt output. The second battery would be charged from a flexible solar panel mounted on my topper and charge controller. I would want the deep cycle battery to be sized for four hours at 20 amps into the standard battery. The solar panel should fully recharge the deep cycle battery in 8 to 12 hours.

I don't think I'll ever do it, but it's a great thought experiment.

I had the exact same idea recently, except that the charged auxiliary battery would also run a couple of small self-adhesive heater pads for engine sump and gearbox - to reduce warm-up times in winter, or in summer come to that.

I'm planning to start with the aux battery and variable input dc/dc converter to see how it works, then add a mains charger for the aux battery. If mpg gains are appreciable I will invest in a roof-mounted solar panel and charge regulator for the aux battery. I'm hoping that simply supplying a slightly higher voltage to the car's normal system (14.8v?) the alternator will 'decide' not to bother putting any amps out, and I will therefore not need an alternator kill switch.

But did you ever make the thing?

[cRiPpLe_rOoStEr]

I might admit I'd be quite afraid to delete the alternator into any vehicle, unless it was either an old-school Diesel or some air-cooled Volkswagen (in such case I could still use a stator from some 4-cylinder motorcycle with distributorless ignition). Some other energy-saving measures such as all-around LED lighting might be also considered to decrease the electric load...

[Ryland]

Ebay has, for $90, 30amp, 36v to 12v DC to DC converters, you would still want a $25 ridding lawn mower starting battery to turn the starter over, but 3 cheap deep cycle batteries would give you alternator-less driving for 100's of miles without dimmed lights.

[JRMichler]

Get a DC-DC converter for 13.8 volts and put in an alternator ON/OFF switch. Lead acid batteries like about 14.3 volts (varies slightly depending on temperature) to be fully charged, and about 13.8 volts for a sustain charge. Too much voltage is bad for the battery, it converts the water into hydrogen gas.

Turn the alternator on about once per week just to make sure it still works and to top off the battery.

It takes almost a full kilowatthour of power to put enough heat into an engine to do any good. That's enough to drain a good sized deep cycle battery.

[paulgato]

Originally Posted by Ryland

Ebay has, for $90, 30amp, 36v to 12v DC to DC converters, you would still want a $25 ridding lawn mower starting battery to turn the starter over, but 3 cheap deep cycle batteries would give you alternator-less driving for 100's of miles without dimmed lights.

Sounds good. Do you have any details of the dc/dc converters?

I was thinking of using a couple of 150w car laptop adapters from Maplins set to 15v regulated output. A similar price to achieve a similar output. (Well, 250 watts seems about right to cover the load during normal running conditions.) Laptop adapters tend to be fairly efficient these days. They take a nominal 12 volt input but I have been told they will accept anything between about 11 volts and 15 volts input. In other words, they are designed to run on lead acid batteries, even significantly discharged ones. Using two such adapters in parallel should work fine I think, especially if they are feeding a system which has a big capacitor attached, in the form of the regular starter battery.

My hope is that by feeding in a sufficient current at a voltage just above the voltage at which the alternator regulates its output (14.4v?) the alternator will be 'fooled' into 'thinking' that the car's electrical load is zero watts and will therefore spin easily around without putting any power into the car. (15 volts is probably just about right, and happens to be a common laptop PSU voltage.) When peaks of current are demanded, the dc/dc converters (laptop adapters) will not be able to provide that much current, so the peak current will be provided by the starter battery, voltage will drop below 14.4v, and the alternator will respond by kicking out amps as normal, ...but for most of the time the alternator will be idle.

Someone, please, correct me if I'm wrong here but I think this would mean that no switches or alterations would be necessary at all, beyond feeding that regulated 15v output into the car's electrical system at some suitable point. When the auxiliary battery becomes exhausted and its voltage drops, the adapaters will no longer be able to maintain the 15v and the aux battery should stop discharging. The alternator will then take over.

Ideally the battery should be about 250Ah. Perhaps 2x 6v 250Ah batteries in series, so each battery can be moved around without lifting gear! But I'll start with one cheap 12v 65Ah leisure battery to test the concept. That will be light enough to carry indoors every evening to put on charge overnight.

15 volts is slightly higher than the normal voltage in my car but should not cause a problem, either for the battery, or the ECU, or for any other component in the car. Indeed, some alternators will output 15 volts anyway. 15.5v might be too high but 15 volts should be no problem.

Someone correct me if my logic is wrong here! What have I missed?

[paulgato]

Originally Posted by JRMichler

Get a DC-DC converter for 13.8 volts and put in an alternator ON/OFF switch. Lead acid batteries like about 14.3 volts (varies slightly depending on temperature) to be fully charged, and about 13.8 volts for a sustain charge. Too much voltage is bad for the battery, it converts the water into hydrogen gas.

Turn the alternator on about once per week just to make sure it still works and to top off the battery.

It takes almost a full kilowatthour of power to put enough heat into an engine to do any good. That's enough to drain a good sized deep cycle battery.

Hmm... Thanks JRMichler. That would work. And yes, I was thinking of 2x 50w pad heaters on both gearbox and engine sump, left switched on overnight. That does add up to about 1Kwhr. Yes, probably not a suitable use for a battery. But leaving the car 'plugged in' overnight could charge the aux battery and warm the engine.

Probably best then to use mains voltage pad heaters reather than 12v ones. It would be nice if a large solar panel could provide enough charge in the battery for the pad heaters but that's probably too much to expect, and in any case that would mean the battery is exhausted by morning, which is the opposite of what you want. But a plug-in overnight thing would work. Even in (the cool, British) summer there should be some gains from starting the day with a warmer engine and gearbox.