Quote:
Originally Posted by brucepick
My experience, in which the 12V lead acid deep cycle really only could power headlights for about 1-1.5 hours effectively, was based on my experience in the Civic. Originally it had too-thin headlight wiring, with too long of a run also. Early in my ownership of it, I remedied that with heavy gauge leads directly from the battery to relays, and more heavy gauge leads to the bulbs. I used the deep cycle battery with that improved wiring configuration.
|
Ah, so you're ahead of me on the wiring.
I am surprised then that you only got 1 - 1.5 hours headlight use out of the deep cycle battery or batteries. I currently have a single 90Ahr AGM battery in the engine bay and I get 5 hours daylight use out of it or about 2 - 2.5 hours night-time use with headlights on. If it's also raining and I'm using wipers then that will drop to 1 - 1.5 hours, especially if also using the blower. The point at which I give up and plug the alternator back in is when I THINK the RESTING voltage has dropped to 12.2v, which should correspond to about 50% charge. I gauge this by monitoring a little volt meter plugged in to the cigarette lighter, although that of course measures the voltage under load, which at 50% charge could be 12.0v or 11.5v or even 11v, depending on the load being drawn at the time. Just to run the engine, ECU, etc. - ie, with no lights on or other 'optional' equipment - takes 6.4A @ 12.5v. Adding sidelights makes practically no difference as all 6 bulbs involved are LED's now, so I often use those as daylight running lights, or even urban (under street lights) night-time lights, as they are quite bright.
I experimented with different headlight bulbs. I ended up using Phillips EcoVision bulbs as they do use about 20% less power than standard (44w as opposed to 55w) but still output a better-than-standard light, and seem to do OK-ish at lowish voltage. Also, keeping the speed right down means the headlight brightness is not so critical, and saves fuel of course.
[CORRECTION: I made a mistake here. I had originally fitted the Phillips Ecovision bulbs, but I found they didn't give out enough light at 12.x volts so I re-fitted a pair of Osram Nightbreaker bulbs, which are marketed as being brighter and whiter at a standard 55w power rating. (I had forgotten I had gone back to the Osrams.) Obviously, you don't get enything for nothing and the brighter bulbs don't seem (anecdotal evidence from the web) to last very long and people seem to have to replace them every year or so. I find that the Osram Nightbreakers) do give out adequate light at the lower, alternator-free voltage, and I believe that at this lower voltage they should also last much longer than at 14.4v. At some point, when I get around to it, I will upgrade the wiring to the headlights - and perhaps then I'll be able to use the 20% lower-wattage Phillips bulbs and still get a good light output - but in practice the Nightbreakers work well already.]
But a battery's Ahr rating changes depending on what load you put on it. You get many more Ahr's out of it if you can keep the load less than 1/10 of it's nominal Ahr rating, so if you have your two deep discharge batteries plus your starter battery (you don't say what Ahr rating they are) then your load should be nice and low compared to the Ahr rating, and you should get WAY more than 1.5 hours out of the set-up, I would have thought. If you had 200Ahr in total then in theory you could draw 20A for 5 hours to get down to 50% charge. 20A on my car would represent engine (6.4A), headlights (10A), and wipers (3.3A). My single 90Ahr battery gives no more than 1.5 hours under that kind of load, but actually less because at that load the battery voltage is pulled right down, and at 50% charge under high load the voltage is too low to light the headlights as I would want (...and under dark and rainy road conditions, I DO want good headlights!) But if I had more/larger batteries - something closer to 200Ahr total - AND if the load were spread between the batteries, the terminal voltage under such a load at 50% charge would be higher - closer to 12v - and the headlights would therefore be brighter.
Just thinking aloud here for my own situation. I had thought that the higher the current capacity of the DC/DC converter the better, but if the aux battery were taking the entire load of the system until its voltage was so low that the DC/DC cut out, leaving the starter battery to take over from there, then each battery would be drained more quickly than 1/10 its nominal Ahr rating. (Unless I had 200Ahr+ of aux batteries!) So maybe it's actually better to have a modestly rated DC/DC converter that will provide something like 10 or 12A, so it will provide ALL the power under low-load conditions and about HALF the power under high-load conditions, thereby spreading the load between the two batteries and keeping the load on any one battery at or below 1/10 of its nominal Ahr rating.
I was even thinking of using one (or two) of these 8.5A laptop adapters, set to 15v output...
15 24V Laptop Car Adapter 8.5A 9 Charging Tips | Maplin
I was thinking of trying one of those out to see if it worked as a DC/DC converter/charger. In theory 15v would be enough to fool an alternator into closing down without having to disconnect it, and 15v would not be too much for the starter battery for an hour or two at a time, given that it would be under partial/occasional load and would have been drained a bit by cranking the engine. And if it doesn't work for any reason then I will have a good car adapter for my laptop, which I am sure to use from time to time.
[Edit: Another consideration is that the larger the batteries in relation to the load, the longer they will last before needing to be replaced. I can't afford the real deep-discharge traction batteries (that can be regularly discharged to 20% without ill effect and will last 10-15 years) and also they tend to be too physically tall to fit anywhere sensible in the vehicle, so I'm going for some AGM 'leisure' batteries that cost just under £100 and come with a four-year warranty. I got a 90Ahr one as that was the largest I could fit in the battery tray in the engine bay, and I shall get a slightly larger (110Ahr?) battery to fit in the spare wheel well. Larger would be better, but it needs to be low-profile to fit nicely in the wheel well. I could fit two of the 110Ahr batteries in the wheel well, but weight is also a consideration, and since 80% of my driving is local and well within the range afforded by the single 90Ahr battery, I think I can only justify the weight and cost of one auxilliary battery of about 110Ahr.]