Episode 10 posted

Well with spring cleaning, I haven't been able to get much work done, but I did procure some batteries (for free!) and am in the process of charging those up. You can see more about it in the video below:



Now for the discussion of batteries and the math I promised.

So here are the pros and cons of the batteries I have. Let's start with the pros:

1. These are deep cycle batteries. That means they can be safely discharged down to a low level (around 80%, as opposed to 50% for standard lead-acid batteries)
2. These are UPS batteries. While their total amp-hour rating is not as much as I'd like, they won't lose as much to the Puekert effect. With lead-acid batteries, they're rated at a discharge rate of 20 hours. So if you have a 75 Ah battery, it can give 3.75 Amps over a 20 hour period. Conversely, you would think that it could give 75 Amps over an hour period. However, as the rate of discharge increases, the Puekert effect causes it to be less efficient, meaning you run the battery down before the full charge is delivered. Typically you lose about 20% of total charge because of this, but because these batteries are designed to discharge over the course of 15 -45 minutes, I expect the performance will work out a little better than the numbers say they should.
3. The UPS connections look pretty convenient. If I can find some of the receptacles for it, I can avoid the time of having to connect lugs to all the cables, and it will give me quick-disconnect ability, which could come in handy.
4. If I can get them charged up to nominal levels, it should give me enough energy to get to work and back, plus a little extra. I've got the math below for that.

Now, time for the cons.

1. The batteries are a little small, only 75 Amp hours (Ah). The more Ah, the more range It won't get me quite far enough to do all the things I do in a day now, but with two cars, my wife can do some of the things, like picking up the kids, that can save me the needed mileage. It will mean changing our routine a bit, at least until I get new batteries.
2. They're not very efficient in terms of weight. A typical deep cycle, 100+ Ah battery would weight around 65 lbs. These weight almost 60 lbs (58.6) and are only about 3/4 that size.
3. They're used, and have been badly maintained. At least one has been allowed to self-discharge down to no noticeable voltage. This means that the life span of these batteries will be fairly short. As they're free, that's not much of a problem, but it does mean that I'll have to replace them fairly soon. Batteries like this have a life span that's measured in cycles, but they will fade over time as well. These have never had a regular discharge cycle, but they're about five years old. I suspect I'll get less than a year of decent performance from them.

So before we jump into the nerdy math part, let's just do a quick sum up. Next week: vacation. After that, semi-weekly schedule, as time allows, and hopefully making breakneck progress after a winter of non-action. We've got the motor and batteries, the first charger, the car, all we have to do is drop the engine, get the adapter fitted, get the dc converter, controller, and wiring. Easy! Well it sounds nice anyway.

Here's that math bits I keep promising.

A lot of people use 300 watt-hours/mile as a general guideline for a small-ish car. I've actually seen people get as low as 260 wh/mi on things like a Geo Metro, and the Geo Storm is just about as light and more aerodynamic. But for the sake of argument, let's say 300 for now.

I travel 7.9 miles, one way, to work. Between this and picking up the kids, that's 7.9 x 4 = 31.6 mi. To feel good about the range, I'd like 35 miles. To get that, I need 300 wh/mi x 35mi = 10,500 wh, or 10.5kwh. Now wh = voltage x Ah. Since we know the voltage will be 96v (we can go up later to 120 if need be) we get 109.375 Ah needed. This is before we take into account that we only want to discharge to 80%, and we may only get 80% of the total because of the Puekert effect.

This means we really need 15.12kwh (10.5 x 1.2 x 1.2, i.e. increase by 20% to account for Puekert and 20% so we don't fully discharge). This means we need 157.5 Ah batteries at 96v. If we up it to 120 (add another two batteries), we still need 120.6 Ah. That's the top the motor and most controllers will handle before you get into AC territory, which will be too expensive and too difficult for this particular project.

However, if I adjust my driving habits and let my wife pick up my kids from school, I can cut out 15.8 miles of my daily drive. That means I really need around 20 miles range, or 6000 wh. To adjust up for Puekert and total discharge, you end up with 8640 wh. At 96v, this is 90 Ah. If we go up to 120, that means I only need 72 Ah.

This means that with these batteries, if I pick up another two, I can get the range I need without having to buy any batteries. And this is assuming 300 wh/mi, which I think will be high for this vehicle, as light as it is. I think it will be more like the 260 I've seen Metro owners quote, which lowers our needs to 260x20= 5200 wh . 5200wh x 1.2 x 1.2= 7488 wh needed total. Divide that by 96v and you get 78 Ah, very close to what I have now. I'd actually get 17.7 miles using these numbers (75Ah x 96v x .8 x .8 /260), which is enough to get me to work and back, plus 1.8 miles extra, and still have 20% available in an emergency. That last 20% would damage the batteries' life, but given their current state, and that they were free, I wouldn't worry about it too much if I had to use it now and then. That could mean another 3.4 miles if I really needed it.

So all in all, the math says that if I can get these batteries to charge, I can get where I'm going, but I might not have a lot extra, depending on how many watt-hours per mile I actually end up getting. I'll definitely be replacing the batters sooner than later, and when I do I'll go with a larger battery, but for now, these should be enough to get me started.