Alternator bypass & engine preheater mod

[redpoint5]

Originally Posted by paulgato

The trouble is I kind of DO want to go for cross-country mpg records on occasion (he-he!) because it's only on longer journies that the highest possible mpg is achieveable, ...but I'll resist the temptation.

Ecomodding is a legitimate hobby, and as long as it brings joy, I see no need to rationalize the time and expense that goes into a project. I'm experimenting with supercaps and LiFePO4 batteries knowing full well that I'll never recoup the cost. Learning and discovering answers to questions that haven't yet been answered on the net is rewarding. It's like being a pioneer on the frontier of technology.

I didn't know about fully-charged lead acid batteries being less efficient than partially charged ones. (Did I understand that right?). Are you saying they are also less efficient on discharge when full?

You understood me correctly. A lead acid battery at near full charge wastes more energy than it accepts. I read somewhere that the same goes for discharge.

Charging and Discharging Lead Acid Batteries

The state of charge of the battery will also affect charge efficiency. With the battery at half charge or less, the charge efficiency may be over 90%, dropping to nearer 60% when the battery is above 80% charged.

http://windandsunpower.com/Download/...Efficiency.pdf

these tests indicate that from zero SOC to 84% SOC the
average overall battery charging efficiency is 91%, and
that the incremental battery charging efficiency from 79%
to 84% is only 55%... Charge efficiencies at 90% SOC and greater were
measured at less than 50%

Lead acid batteries need to be kept at full charge to maintain longevity, but charge efficiency at near full charge is abysmal. For lead acid batteries, efficiency and longevity are at odds.

For this reason, I have become interested in exploring alternatives to lead acid batteries.

[paulgato]

Originally Posted by redpoint5

Ecomodding is a legitimate hobby, and as long as it brings joy, I see no need to rationalize the time and expense that goes into a project. I'm experimenting with supercaps and LiFePO4 batteries knowing full well that I'll never recoup the cost.

Aha, yes, I understand that LiFePO4 batteries are more efficient and don't particularly suffer if operated at lower states of charge.

From what I understand, lead acid batteries really suffer a lot if not recharged to 100% between cycles, but perhaps this charge acceptance inefficiency at near full charge means that I would be better off not allowing one battery to actually recharge another while driving. With that in mind, designing a system so that in normal use BOTH batteries are being discharged might therefore be more efficient. So if using a DC/DC converter it would be better to use a relatively low-power one. Or else just use a simple parallel connection with no converter.

(But wait, ...if efficiency falls as charge nears 100%, that is also when the actual charge RATE reduces right down, so the absolute energy loss is perhaps not that great.)

At the moment I'm thinking of a no-compromise system which also won't be expensive:

1. For normal local driving I use my existing two good quality deep-cycle-capable AGM batteries in simple parallel arrangement.

2. For longer journies I drop in an additional 90Ah AGM 'leisure' battery (a free warranty replacement for the cheap one I wore out with three months of daily deep cycling) I plug that into the installed 12A DC/DC converter to take the strain off the other two batteries.

I won't worry too much about how long the drop-in cheap leisure battery will last as it will only be used once a month or so. This arrangment means that if there is no means of plugging the car in to charge at my overnight location (like city on-street parking for example) I can either keep the DC/DC converter running overnight to put as much as possible back into the other two batteries, or else carry the leisure battery indoors and use a portable charger to recharge it overnight. (Or both: I can leave the converter running a few hours on arrival, and then at night carry the leisure battery indoors to be recharged.)

This will also mean I am maximising mpg on local drives as well as on long dirves as I won't be carrying that extra 25kg unless it is being used.

Originally Posted by redpoint5

For lead acid batteries, efficiency and longevity are at odds.[/B]

For this reason, I have become interested in exploring alternatives to lead acid batteries.

Yes, I can see that. I'll be interested to know how you get on with the LiFePO4/supercapacitor arrangement. That has to be the future. I was thinking of locating all batteries in the rear and just having a parallel-connected supercapacitor array in the engine bay for engine starting.

Actually I think the future is also to have solar cells built into the paint layer of vehicle body panels and/or the use of photovoltaic glass. That would happen at manufacture of course. Most vehicles are being driven less than 5% of the time and spend all day soaking up solar rays. Again, batteries which are efficient and long-lasting at partial states of charge would be needed in order to take full advantage of solar cells.

[paulgato]

Just to give you all some idea of how this two-battery system is working so far, I have today driven 350 miles or so, with 12.5 hours of drive-time, and all without using the alternator.

The first 7.5 hours of that was from the East coast to Oxford in heavy traffic, with some hold-ups, in daylight with no lights on. Approx current draw: 8A.

I then had a 75 minute break at home, where I plugged the car into the mains, recharging the batteries at a rate of about 35A.

After that 75 minute 'tea, food and charging' break I drove another 5 hours to Bristol and back, this time in the dark with the headlights on, and also with CD's playing loud on the car stereo to keep myself awake. Estimated current draw at the batteries: 17A.

On my return home, the DC/DC converter had almost shut itself off (had cut back from 12.5A to about 2A) and the 'donor' battery had got down to about 40% SOC, with a resting voltage of between 12.0v and 12.1v. The Odyssey starter battery however still had plenty of life in it and was sitting at a resting voltage of 12.5v a couple of minutes after switching off the engine.

This is much better than I had expected, ...which is slightly disturbing as it might mean my calculations about current consumption are out. Alternatively (and I think this is more likely) the 'donor' battery - the Yuasa YPC100-12 104Ah deep cycle AGM - is performing significantly better than spec.

Earlier today I filled up the tank with about 58 litres of diesel, and fuelly.com reports that on that last tankful I drove 1150 miles at an average of 89.51mpg Imperial. I do drive slowly, but for a fairly large and heavy estate car I reckon 90mpg overall average is pretty good. (The odometer under-reports distance by 1.5% so the true average was over 90mpg.)

[ctgottapee]

Have you worked out exactly where all your juice goes? Would be interesting to see the stats on that.

[paulgato]

Originally Posted by ctgottapee

Have you worked out exactly where all your juice goes? Would be interesting to see the stats on that.

Yes, towards the beginning of this thread I recorded the current consumption of all equipment on the car.