[paulgato]

Originally Posted by brucepick

Paul,

I owe you a small clarification on the battery project.

The 40 AH LiFePO4 was capable of powering the car (minus headlights) for 3 hours, with minor occasional use of fan or wipers. Nominally rated 12.8V. With an allowable 80% drawdown of battery capacity and a derived approx 10A load due to running the basics (computer & fuel pump), the 3 hour life is about right.

Sounds about right. These LiFePO4 batteries seem much better than Lead Acid in every way really. Just expensive.

The ~100 AH lead acid, nominally 12V of course, was effective for headlights for only about an hour. This was mostly because 12V isn't really enough for headlights, they need about 13.5-14.5V, subject to user preference and opinion. Of course voltage dropped below 12V pretty quickly. At night the illumination from slightly reduced voltage just wasn't ideal.

I realised I was wrong in an earlier post. I am not using those (44w) Phillips EcoVision headlight bulbs at the moment after all. I found they were a little too dim at low voltage. I was intending to up the voltage a bit with the headlight relays and heavier cable and then re-fit the EcoVisions to save a bit of power, but haven't yet got around to it.

I had forgotten that I had ('temporarily' - ha ha) fitted Osram Nightbreaker bulbs, and those do seem to work well at low voltage. They are sold as being whiter and brighter than normal, although they only use the standard 55w of power, so remain legal and won't overload any circuits. The 'net wisdom' on the Osram Nightbreakers is that they do work well but tend to burn out in less than a year, so work out expensive. I find that they work well for me at 12.5 volts and are acceptable in brighteness and colour at any voltage at which the battery is still above 50% charge. (In fact they are acceptable in brightness even if I have discharged the battery to the point where the engine will not start - as I found out the other night! I had a spare battery with me so I was OK.)

I think my comments about the headlights being alarmingly dim at 11v must have been based on either standard bulbs or the Phillips Ecovisions because on this last trip (150 mile round trip, half in darkness) I didn't once feel the need to stop and reconnect the alternator because the headlights were getting dim, even though the voltage had dropped way below 11v, whereas I know I've been compelled to reconnect the alternator due to dimming headlights once or twice this last Autumn.

I guess the Osram Nightbreakers (and similar bulbs) have been designed to maximise light output at the cost of a shorter life. Run at 12.5v they seem to shine pretty much like a normal bulb would at 14.5v, and at 12.5v they will probably last as long as a normal bulb too. I think that since I am almost not using the alternator at all now (so my mpg is no longer directly affected by current draw) I shall stick with the 55w Osram Nightbreaker bulbs, as they do seem to work well at lower voltage.

The other limiting factor for the lead acid pack was that I wanted to drain it down only about 50%, to preserve its longevity. Lithium, on the other hand, can safely be drained to about 20% of it's rated capacity.

LiFePO4 batteries seem way better than lead acid. A 50Ahr lithium battery has the same useful capacity as a 90Ahr lead acid, will last many times longer and probably weighs less than a third of its lead acid aquivalent.

Thus my interest now in several booster converters in parallel, to power the headlights and probably also brake lights and fan.

You might want to try the Osram Nightbreakers, or similar brightness-boosted bulbs. It may be all that's required, and would save a lot of hassle if it works.

As for the brake lights, I can heartily recommend swapping the incandescent bulbs for red LED bulbs. LED's are not voltage sensitive in the way that filament bulbs are. The LED's I fitted use 1/5 the power, are much brighter and 'redder' (the red light filling the lens evenly rather than being brighter and paler in the centre) and they switch on instantly, so giving drivers behind an extra 1/3 second for braking. They also have an expected life longer than the expected life of the vehicle, so hopefully I'll never have a brake light fail on me again.

I'm certain I could have powered the stereo off the lead acid for many hours; it doesn't need ~14V to satisfy my entertainment needs. However performance stereo buffs have been known to use multiple 14V batteries in parallel for earth-shaking bass.

14v batteries are available, if expensive, and alternators can easily be 'tricked' with a couple of diodes into producing a higher voltage to charge them, but 14v mains chargers are not so easily available.