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Discussion Starter #1 (Edited)
My drive for work is 60 miles round trip. I have not been able to charge at work and my maximum range was 62 miles. This ran the battery down too far to leave a safe reserve of energy. I needed a little extra power to get around and to run lights, heater, etc.

In the photo you see a homebuilt (DIY) battery pack with 220, C size, NiMh rechargeable cells in it. There are 2 banks of 110 cells. The 2 banks are connected in parallel. Every 2 series cells has a power diode reverse biased across them. The voltage of the pack is 135vdc. This gives the much needed extra miles that I needed. The car now charges at a rate of 3Kwh on level 2 charger and will fully charge in six hours or less. I have secured the pack in the trunk for testing but, it will be mounted inside a box under the car.

I managed to get the cost down to approx. $400. by sorting through almost 300 really cheap cells from China to find 220 good ones.
The power curve of the NiMh pack is almost the same as Lifepo4 and it will deep discharge and charge 1000 times or more. This pack weights only 18 lbs.
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So in theory how hard would it be to make a pack large enough to extend the range on a hybrid? Lets say a Yaris HSD which only has a 1kWh battery to start with - how hard/expensive would it be to buy decent cells and make up another 1kWh pack to effectively double it's ev use?
 

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Discussion Starter #3
So in theory how hard would it be to make a pack large enough to extend the range on a hybrid? Lets say a Yaris HSD which only has a 1kWh battery to start with - how hard/expensive would it be to buy decent cells and make up another 1kWh pack to effectively double it's ev use?
Top quality NiMh cells with a capacity of 5000mah and 1.2 volts per cell will cost just under $4 each if purchased in bulk. It would require a minimum of 167 of these cells in some configuration to make a 1kWh pack. Two packs made with 110 series cells could be put together in parallel as, I have done and, this would produce 1.35kWh at 135vdc max. It could weight 36 lbs. and cost about $880.00. This might not be the least expensive option. I have a 124 volt system and the reason that I built the NiMh packs with 110 cells was to get a peak voltage well above the peak for the rest of the Lifepo4 batteries. The packs never charge above 126volts so there is no danger of ever over-charging the NiMh cells. The manufacturer states that these cells can be used to build packs up to 24vdc. I understand why; with so many cells in a series string there is a high risk of failure. I carefully balance charged all the cells and wired a power diode reverse biased across every two cells in each pack. If any one cell were to go bad, the diodes are the only thing that would allow current to bypass the bad cell and prevent excess heat in the pack. So far, I have not had any trouble but, Winter is on the way and I do not know how well the packs will hold up in freezing temps. I will encase these packs with electrically insulating materials that are flame retardant, corrugated plastic to absorb impact damage, and fiberglass to make a rugged package with mounting flanges. The dimensions of the packs are a perfect fit under my car in the space that the gas tank once occupied. This has been an experiment that I have performed with caution. Anyone attempting to follow this example must do so at their own risk.
 

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OK, this is where I show my ignorance beyond the real basic understanding of rechargeable battery technology (and electronics)... if you have so many in series and the voltage is 100x that generated by a single cell, is this transiting voltage (and current) not harmful to the cell? (In battery chemistry terms I mean).
 

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So in theory how hard would it be to make a pack large enough to extend the range on a hybrid? Lets say a Yaris HSD which only has a 1kWh battery to start with - how hard/expensive would it be to buy decent cells and make up another 1kWh pack to effectively double it's ev use?
Easy but pointless, the toyota hybrid system will not "know" there is more capacity available and so will not use it. Lots of information about this (and products to trick the hybrid system) out there if you start googling.
 

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Discussion Starter #6 (Edited)
OK, this is where I show my ignorance beyond the real basic understanding of rechargeable battery technology (and electronics)... if you have so many in series and the voltage is 100x that generated by a single cell, is this transiting voltage (and current) not harmful to the cell? (In battery chemistry terms I mean).
The current might be harmful if too much were drawn. These cells normally can be discharged at a rate of 15 amps each but, I am hesitant to draw more than 10 amps. I have the two packs charge limited to 5 amps and the packs discharge limited to 20 amps. I worry more about the tabs welded to each cell than about the cell itself because the power or, watts (volts x amps) is so much higher than normal. I was pleasantly surprised to see an improvement in range and none of the cells leaked or overheated during charge or discharge.
I believe that this is only working well for me because the NiMh batteries are supplying a very small percentage of the energy that runs the car. The car might draw 500 amps at 124 volts but, it gets most of that from the main Lifepo4 batteries. My objective has been to extend the range by, as much as, 5 miles.
 
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