I have been planning a little project and like to get some expert opinion before getting too far into it. While most of us would like to have an Li conversion, few are willing to take the plunge and spend the money. But really what most of us suffer from is range anxiety isn't it? So why not carry a spare battery in the trunk? Here's the approach.
I have been playing around with RC Lipo batteries for some time. They can generate 20C easily and have come down in price substantially. You can buy a 22v 5200ma pack for 46$ with shipping. These weigh 700g. One needs 7 of these in series to get 156v. This yields a pack with nominal voltage of 42cells at 3.7v each or 155V. This pack will connect through a diode protected cable to the battery. I already have a connection port to use a Freddy like charger to balance the pack. Once you run low on battery, you engage the spare battery via a marine type 20a breaker and up to 700 watt of power will be available. Since the batteries connect via a diode, it's like having a 20ah charger connected to the battery. I figure that the resistance of the battery is about 1ohm and hence with a 20V differential the maximum current will be about 20a. At this rate the transfer will take a bare 15 minutes to complete. As far as extended range, 700watt should yield about 9-10 extra miles. This setup would cost about $350 dollars. One can parallel charge the 7 packs via a cheap RC type charger in about 5h. Since the batteries are Li, they will hold the full charge for several months. What are the experts saying? Bring it on!
Reserve Battery
Mon, 06/13/2011 - 18:57
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Reserve Battery
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So the diode can limit the current running from RC LiPos to NiMH pack? That's nice. But for stationary, I'd limit the current to 12A - as rated for the Vectrix NiMH cells.
For short bursts, 12+A is ok, but for longer charging (15 minutes as you describe), it will heat the NiMH pack and even V's plenum fans won't be running.
However, if you consider driving, while charging, you might consider, that 42 cell LiPo pack, at 120V (empty Vectrix), will get your LiPo cells down to 2.8V/cell, which is quite low.
P.S.: Which 5Ah LiPo cells do you have? For my T-REX 600, I have old Flightpower 4900mAh and 5000mAh 6S LiPos, but I heard that Rhino cells from HobbyCity are ok.
Edit: Oh yeah, 42 Lipo cells will get you 42 * 4.2V = 176V when fully charged. If I were you, I'd use 37-38 LiPo cells.
I would be using this while driving. Most of the current would go straight to the Motor not the battery. The 42 cells would ensure that at 120V when the V is near dead the low voltage for the Lipo pack is 2.85V, well within the specs for these kind of packs. The higher initial voltage ensures that the pack could be used even if the main battery is only partially depleted. Finally, this arrangement could also be used in conjunction with the LI Vectrix which also has 42 Li cells. As to the diode, I use a diode used in solar applications. You can get them rated at almost any amperage and yes, the diode rectifies the flow of current whether from a poser supply or battery.
I use a similar approach - but much smaller.
A set of 8 AA NiMH cells gets charged via the 12V outlet in the glovebox (diode protected) whenever charging and/or riding.
This 8s pack can be used to charge up to 8 of the stock cells initially. Because I have arranged them in order of capacity, I can first charge the last 5 cells, then 4, then three cells, then two and one cell as the voltage of the auxiliary pack drops. This will give a little bit of extra range, but not all that much.....they are 2.8Ah cells.
Unless your battery pack is very capacity-balanced, you are probably better off by concentrating on the weakest link (cell). But it's a lot more work than just charging the whole pack. But, it's a lot safer, too, once it's all installed. While your stock pack connections are diode protected, the auxiliary pack is of course not diode protected and just as lethal as the stock battery. You could keep it separated into modules until actually needed to make it safer.
This information may be used entirely at your own risk.
There is always a way if there is no other way!
Mike,
The plan is to use PowerPlug connectors for each of the battery, with the final assembly having 7 negatives and 7 positive leads each. The plug connecting it to the bike will tie the pos and negatives in series to achieve a 156V when plugged into the bike. Once disconnected they each have 22.2V. The charger plug has all positives and all negatives in parallel. This way the pack can be charged with a regular RC charger at 22.2V in parallel. To keep the cells balanced, the balancing taps from each pack are also connected in parallel during charging. THis way one always has a well balanced auxiliary pack. The only time high voltage is present is upon connecting the pack to the bike. This would only be done on demand. your thoughts?
Vectrix has LiFePo4 chemistry, which usually is 3.2V nominal, RC LiPos are 3.7V nominal.
Good point Andy, since these packs come as 6 cell packs, should I use 6 packs I.e. 36 cells? Nominal voltage would be 133.2 with peak voltage of 151.7? The next step up would be 42 cells. What do you think?
I think that is a good number of cells. Also, they will last longer, since the cut-off at 118V will give you 3.27V per cell cut-off.
I like the idea. But not that familiar with these RC packs. How many charge cycles for these LiPo packs?
How well would they handle the possible heat build-up within a truck box parked in the sun.
Motorcycles: 2011 ZEV Trail 7100, 84V, 60AH, 60+mph, Cycle Analyst, TNC throttle, modified charger. 2013 Kymco GT300i
Bicycles: 2017 Sondors Thin
Cars: 2016 Leaf SV, 30KWH pack. 2007 CR-V
Solar array: 5KW. Cost per lifetime KWH produced $0.073
Bi
They should easily do 300-500 charge cycles. They are unfazed by temperatures up to 60-75C !
Maybe, but below 20C discharge current. My helicopter packs lasted about 200 cycles.
Discharge would be limited to much less. I will try this out on a smaller scale. Just received my 30a breaker. I was expecting this battery to max out at 30a I.e. 6C. It will be connected via a 150v 30a DC breaker through a 40a diode. The Vectrix NIMH battery can handle 1C, I.e. 30ah charge rate.
I don't think cycle life is a main concern, because this is meant to be a "reserve tank" that is rarely, if ever, used. If the extra battery allows you to ride a few extra kilometres only 100 times, that should really be sufficient for the entire service life of the main battery.
This information may be used entirely at your own risk.
There is always a way if there is no other way!
HarryS,
Let us know how the project goes. Instead of using the battery pack as a battery charger, I was hoping of using a pack like this to run my 72v 6100w e-scooter off of in emergencies for 5-6 miles. My concerns of using it as a charger are charging energy losses and time to charge; where as, with it directly connected it would be ready to go at a flick of the circuit breaker. I have a circuit breaker under the seat to disable the main pack and thought of adding another circuit breaker to enable only the LiPo when needed.
What LiPo brands provide a good value 5ah at continuous 20-21C. I was looking at some of the 30C max, 25C continuous models as a possibility for a 20C current. Read some bad reviews of the Winforce models, so might look at Turnigy or Zippy. Most of my riding is in the 5-15C range but sometimes I would need the full 20C current.
Motorcycles: 2011 ZEV Trail 7100, 84V, 60AH, 60+mph, Cycle Analyst, TNC throttle, modified charger. 2013 Kymco GT300i
Bicycles: 2017 Sondors Thin
Cars: 2016 Leaf SV, 30KWH pack. 2007 CR-V
Solar array: 5KW. Cost per lifetime KWH produced $0.073
Bi