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Old 01-12-2011, 10:04 AM   #20
L3Research
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Default Roll your own battery pack?

Quote:
Originally Posted by MTOBATTERY View Post
Since the batteries attach to the BMS in 12V clusters he would have to add cells in multiples of six. Unbalanced battery clusters would result in reduced performance if a single cell were added. We have done the same thing with small SLA and LiFePO4 batteries. It can be done but the like always the OEM charger is not compatible.
My confusion, here. Ground loop was talking about NiMH, and I had convinced myself that Safions were LiPo, when it seems they are LiFePO4. The hobby packs are made from stacks of prismatic cells. Adding another identical cell in series with those 1P packs should not unbalance anything. I'd try carefully to open up and remove a cell from one of the $8 packs (you did buy extras?)

Quote:
Originally Posted by Ground Loop View Post
Correct. I dare not use the Magnetek external or the Segway internal chargers with these batteries -- I'm certainly it would blow them up with a NiMH charge profile.

I'm removing the battery cases from the Segway, and then removing the 3S Lipo's from the batteries to charge on a bench... that's 12 3S batteries, for anyone keeping track.


As you know, the Segway effectively has four strings of NiMH batteries, two in each battery pack. Each string is made up of three 10-cell strings.
So for one voltage rail the Segway has 10+10+10, four times.
It *does* monitor the voltage of each of those 10-cell strings, so I can't spoof it by replacing all three with one large battery pack.


Instead, I have 3S + 3S + 3S in LiPo, each one replacing a string of 10 NiMH.

I would very much like to try "3S + 4S + 3S" to see if the Segway complains about inequality, or abuses the 4S pack, or just corrects the battery gauge.

Given the discharge curve of LiPo, I'm not all that concerned with the low battery bars, since it will just park at "two bars" for most of the useable range.

I have only taken apart the Lithium pack. Thanks for the details and the photos. You might spoof it effectively if you were to feed it N fake voltage signals, or copies of one rescaled voltage value for the whole pack. If the goal is to keep your lithiums from undervolting during a glide, the stock NIMH circuit may not be calibrated to save an 11.1v cluster if it expects a 12V cluster as I think Jason wrote.

If the Seg's undervolt threshold is 0.9v per cell, then it will stop you (presumable shake stick and shut down) at 7.2v per cluster. Lithium Polymer cells, I've read, have a cut off at 2.5v per cell or 7.5v per hobby pack. If the Seg's cutoff is 1v, then you are fine. May I suggest, if you haven't seen them, inexpensive audible alarm mini-boards for your packs. Some can be seen in the accessories section at Hobby shack.

Do you or anyone else know what info and in what format the Segway control board "must"get from the battery control board? Aside from battery power, I only saw four pins leaving the battery housing; anyone know what they have on them? If we have clusters of 12v, then these pins can't be sense pins for the 6 clusters. Please don't tell me there is a CAN channel here. I'd just as soon be rid of the batt control board if it isn't performing a charging function, and the main board can be convinced to do without it.



Quote:
Originally Posted by Ground Loop View Post
First, this is what the PCB inside the battery looks like:


It's quite a bit more than I was expecting, and dashed any hopes I had of a full roll-my-own battery pack. Wow.

Notice the steel battery tabs at the top, which are bent over the top of the PCB. For my second battery pack conversion, I thought I'd try to be cleaner and replace the battery tabs entirely, by soldering the 12ga wire to the PCB.

That wasn't an effort I'd like to repeat. It required drilling holes in the back of the battery just to get to the solder tabs:


The resulting battery looked fine, and had fewer sharp edges, but was a more strained fit with the large wires coming over the top:


And the Zippy/HobbyKing XT60 connectors are actually pretty awesome, especially since they come on the batteries already. HK sells the female connectors, with leads already soldered, so this is pure Win. I like them.

I think I'm going to make a third assembly, using XT60 connectors (again) and soldering to the cut-off battery tabs. Then I'll have all 12 batteries on the same connector, which would be nice for massively-parallel charging.

If I was doing this over again, and not so bent on using cheap $8 batteries, I think 4000mAh or even 5000mAh of the right dimensions would be the way to go.

I'd like to find a way to bring the required leads out of the Segway for charging without so much screwdriver time, too.
I think the right number of 3.7v cells in series for the 12.0 firmware is likely 19, not 18 and not 20. Prime numbers can be inconvenient ;-)

If you build a third assembly, let me offer you use of my Fein cutting tool and help you part the top at the original weld line. You should be able to get at the old charge board this way. I have been searching for hobby-priced cells that will fit in a larger Lithium Segway battery shell I recently opened. 10Ah per shell may be possible - 1.5kWh per Glide!

Either way, I have an electric bike project and want to buy some of the $8 packs, so, If you want to combine shipping, let's talk. I also have a battery test unit and some data logging capabilities in my shop in El Cajon, in case you are interested.
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