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Monday, December 14, 2009

More Batteries, More Boxes, More Chargers.

Recently donated was another pair of old wheelchair batteries in their box with a charger. All of these are useful, as the batteries are the same size as my 12V 17Ah, but they are 20Ah. The box is an interestingly useful shape, split to lock in over a central electrical connection, but in this case it is wide enough to fit over a top tube! The charger is a 4A 24V charger almost identical to the 3A charger I already have.

First, there is the possibility now to hook up both chargers like this:

if I change the pack to a 48V using four SLAs instead of the three I currently use. I was planning on that anyway, with the fourth to go in the rear triangle once the suspension is done. This would let me simply plug in wherever, and charge up the batteries unattended with the automatic chargers' own 3-stage normal charging, instead of the two ways I do it now.

At home, for overnight charging, I set the big heavy Sorenson for 44.5V and max current, then plug it into the pack in place of the motor controller.

On longer trips where I can recharge along the way, I carry the 3A 24V charger, which is only a couple of pounds, along with an Anderson that has a jumper wire across it. Since it only charges two at a time, I unplug one battery and plug in the jumper in it's place, then plug the charger in place of the motor controller. Every 30 minutes to two hours, I move the jumper to a different battery connection, so that all three get a part of a charge cycle. It is not optimal by any means, but it extends my range some.

Thew new stuff:

The new batteries are 20Ah,

But it is the same size and about the same wieght as the 17Ah:

The battery box opened up

and the handle on it's top is removable.

If I cut out the part where the charger connector went (an XLR), up to the bottom lip of the cover, then put an upside-down "U" of aluminum sheet in there to help restiffen the case, the box can be placed across the top of the top tube just behind the headstock, at the front of the bike.

This would put about 30 pounds up there, so it will definitely change the way it handles, and will probably take out most of the shock absorption the shock fork gives me now.

If I used this box I would put the fourth battery plus the rearmost of the three existing ones in this, moving much more weight up front to balance the bike, which would help especially when I am going to carry cargo.

However, I could also make it so these are optional batteries only used for additional range. To do this, I would keep all three existing batteries where they are, and add two new ones in this, plus a third new one in the rear triangle. All three new ones would be wired in parallel with the existing ones, as a second 36V string. A mostly-removable wire harness would need to be made that leaves extra Andersons hanging at front and back always on the bike for the other batteries to just plug into, so that I could add or remove the extra ones quickly as needed.

This would still leave me with the problem of charging them, as there would still be sets of three. However, they can be charged in pairs at least, so an automatically-rotating charger is possible but would involve heavy-duty relays/contactors or cabling (which would also be required to auto-rotate as I currently do manually, as motor power would have to flow thru some parts of the current system).

So, to charge them in pairs like this, I could simply use a timer that every 10 to 30 minutes or so would switch from the first pair to the second to the third and back to the first, switching a set of high-power relays in at least one case, though low-power could be used for the other two pairs.

For the "bottom" pair in each triplet, simply switching a low-power relay on across each one pair to engage the charger would work.

For the "top" battery in each triplet, it is harder, as they cannot directly be connected in series. First their parallel-ish connection (both positives wired together) must be changed to series (positive of one to negative of second, opposite pos/neg then connected to the charger).

That one requires a heavy duty contactor to open between the positives of each battery and the rest of the bike. Then a second (low-power) contactor to close that connects the batteries in series, and also to the charger.

The heavy-duty contactor is the issue, as it must be able to handle 100+ Amp surges during motor operation (though it does not ever have to make/break during load).

So it is much easier to go to 48V and get only one extra battery's worth of power, for now. At least until I get the Li-Ion packs finished (they are not yet started for lack of a BMS).

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