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[electrictwowheeler]

First off, I am not suggesting anyone do this themselves. I'm posting it for informational purposes only!

I had some old battery packs I built for my Brammo Empulse years ago. I used them as range extender packs carried in the side cases. Thought I'd try to build a range extender that would fit in the tank storage area of my SRS which has the new battery. It works! I could only fit 1.5 KWH using 168 2.5 ah 18650 cells, 6p28s configuration. The easiest place to tap into the battery is where the buss bars connect to the controller input under the battery. The extender pack has its own contactor because if you put power to the controller with the bike turned off you will bypass the pre-charge system and the high current inrush can damage the controller and/or the extender pack!!! On the Gen 3 bikes the DC-DC comes on after the main contactor closes when either keying on the bike or starting a charge, so the extender pack contactor is energized by the 12v power plug located under the tank just forward of the tank storage area, very convenient! So the extender and main pack are only connected to each other when the bike is keyed on or plugged in. This won't work on Gen 2 bikes!

Caution !!!  When you install the extender, you MUST MAKE SURE the voltage is within 1 volt plus or minus of the main pack voltage !!!!!!!!!!!!! The greater the voltage difference is between the extender and main pack, the greater the current flow from the higher to the lower voltage pack will be.      The extender BMS has short circuit protection so might give some protection but I have not tested this theory. You can remove the extender any time because the packs will be the same voltage. So as long as the extender remains installed, everything works as usual. I wouldn't recommend this as a permanent installation because if the bike sits for a long time the voltage difference between the two packs could get to a dangerous value. I plan on using it on longer trips only.

I get about 10 to 15 more miles of range on a 100% charge. When you first install it, the range estimate will be low but will correct after some driving. When you remove it the range estimate will be high but will correct after some driving.

[Specter]

i can't stress enough the voltage difference he states.  even a volt is a bit high.  if you are more than a volt or so off you can send 100s of amps where it don't belong, and even though the other device may have short circuit protection, do you want to risk it?  do you really want to have to tear something else apart because you popped an internal something, or cooked a link or smoked a trace etc etc.  even if you are only pulling like 5o or 60 amps, that power is pretty much purely reactive, it's doing no work, doing nothing good for anyone or anything.

aaron

[MrBlc]

Chiming in with the same message. The resistance of these batteries are in the milliohm range.
Every bit of preparation matters.
Do the work of making sure the voltage is as close to the monolith as possible at connection.

[ESokoloff]

How difficult would it be to make/purchase a controller that would not allow the pony battery contactor to pull in when voltage delta was greater then 1 volt?
I’m assuming that the smaller battery is most likely to be the one that’s lowest after prolonged idle time.

[electrictwowheeler]

That is a great idea. I don't have the expertise or knowledge to build that or answer your question. No doubt that it is possible and would solve the safety issue. The Zero power tanks do that. Maybe someone with the right skills can chime in. I did test the inrush current today with the main pack 2 volts higher than the extender and it was only a peak of 6 amps and then slowly tapered down from there. The extender is only 9% of the combined packs capacity.

I'm not sure which pack would self-discharge first. I would assume it would depend on the health of the cells and what the current draw of the BMS is, or in the case of the bike, any associated electronics that are drawing current while it's turned off.

I should add that the circuit is protected by a 60 amp fuse so there will not be hundreds of amps burning everything up. The BMS is rated at 60 amps continuous, the contactor at 50 amps continuous, the wire has an ampacity of 80 amps and the cells are rated at max 10 amps each discharge so 60 amps.  These are LG M26 cells, 2.6 ah each, 6 in Parallel.
 
There is an issue with cell balancing. My bike has the replacement BBC battery capped at 12.6 KWH. They do that by limiting the charge voltage to a bit over 115v. The extender BMS is set up to charge to 4.2 volts per cell, so 117.6v total. it will have to be fully charged off the bike occasionally to retain balance.

[Specter]

they make a bunch of ssr's that would do exactly that.  you could also get a cheap ass controller to look at the voltage and put out a fleapower  signal to drive the ssr, or use standard 4-20 control or 1-5 if you lost the resistor .  this is more crap in it though making things more complex.
you could repurpose a simple ass balancing circuit to be the pilot, steal one out of a bms on a battery thats tits up, hook it across the 2 packs if there is more than 50mv of volt diff it'll be active keeping the ssr open, when the voltage drops in specs it lets the ssr go., .. a few wires and the main battery as the pilot and you are in business.  if you really want to be geeky use the balancer as intended so it slowly brings the batteries equal and when in specs turns around and parallels them properly via the ssr.    how nerdy do you want go get with it?

aaron

[ESokoloff]

That is a great idea. I don't have the expertise or knowledge to build that or answer your question. No doubt that it is possible and would solve the safety issue. The Zero power tanks do that. Maybe someone with the right skills can chime in. I did test the inrush current today with the main pack 2 volts higher than the extender and it was only a peak of 6 amps and then slowly tapered down from there. The extender is only 9% of the combined packs capacity.

I'm not sure which pack would self-discharge first. I would assume it would depend on the health of the cells and what the current draw of the BMS is, or in the case of the bike, any associated electronics that are drawing current while it's turned off.

I should add that the circuit is protected by a 60 amp fuse so there will not be hundreds of amps burning everything up. The BMS is rated at 60 amps continuous, the contactor at 50 amps continuous, the wire has an ampacity of 80 amps and the cells are rated at max 10 amps each discharge so 60 amps.  These are LG M26 cells, 2.6 ah each, 6 in Parallel.
 
There is an issue with cell balancing. My bike has the replacement BBC battery capped at 12.6 KWH. They do that by limiting the charge voltage to a bit over 115v. The extender BMS is set up to charge to 4.2 volts per cell, so 117.6v total. it will have to be fully charged off the bike occasionally to retain balance.

Fuses lag a bit in protection as they are a melt to protect device so high current passage is possible (albeit very briefly).
Time delay fuses extend this time as they allow for inrush current (as apposed to sustained).

Another idea for cross current protection is to install diode(s) to prevent back flow.
To allow on bike charging, wire it/them through the N.C. contact of a double throw contactor(s).
Figure out how to energize the bypass contactor(s) when in charge mode. The N.O. contacts would bypass the diode to allow back current for charging.

[T.S. Zarathustra]

As so often happens I tend disagree with some other people who voice their concerns.
If you are worried, use a two stage switch and a headlight bulb (or 10 ohm 10 watt resistor (Edit. 1 ohm 10 watt resistor )) when connecting the batteries together. First connect them through the bulb/resistor. That gives you current limitations. After few seconds connect them directly. If you want to be fancy you can connect an ampmeter to the first stage and only connect directly once the current is below couple of amps.
I would keep the batteries permanently connected together. Then discharge doesn't matter, the voltage stays equal.
One volt difference is fine, also two or three.  As soon as the contact closes the voltage will become same between packs. Inrush of current will drop fast and be pretty much safe level within few seconds. The batteries Equivalent Series Resistance (ESR) basically does not allow enough current to flow to damage the batteries at these low voltage differences. As long as one battery is not fully charged and the other one is empty.

[Specter]

a battery that can hold several kilowatts of stored energy, even with a modest 2 or 3 volt difference can push hundreds of amps across the connection, and continue for a while until its stored chemical energy has exhausted into the other battery to bring their volts to being equal in which case the current will slowly stop flowing as the voltages equalize out.  this will severely stress the batteries, the bms and other components.  lithium does not like this, at all.

you have to remember with lithium.  at the top and bottom of the curve, yes a few volts, holds little energy, but lets say from 50 to 80 percent charged, a nice middle range, there's a lot of power in those 2 or 3 volts of drop.  when you hook all those batteries up in series, you have the ability to push some hefty amps, for a while, to equalize it out.  even a 20 second inrush can be enough to pop internal protections or cook stuff that isn't designed for that kind of load over time.  high current can generate high heat real fast too.  if you have a weaker battery, that may cause a hot spot, which is enough to start a slow cook off, it overheats there, it can't get rid of the heat fast enough, starts stewing,  then finally cooks off.

ive seen more than once where a tech will touch something, huge current thru pack for a second or two, he jumps, breaks connection.  you ok??  yah, what the hell dumbass... derp. is the pack ok... it looks ok,  well keep an eye on it...... feel it.... it's a little warm but nothing bad. ok...  5 minutes later the thing starts smoking, the packs are swollen and its growing, ok get that fucker outside now!   a minute later its on fire as the cells sympathetically pop off one by one.

now, can you get away with it, yes, depending on your situation it may not be a huge issue with your setup, but this requires you to have the brains to understand YOUR setup and know it's behavior, and that it is safe to do, but overall, is not a good habit to develop.  The idea of using a light bulb or some sort of current limiting device in line is an excellent idea, that many use all the time.  this lets you see the current flow as well as volts, so you can monitor and after say 5 or 10 minutes when the batteries have equalized, then you can directly connect them w/o a limiter and they'll be fine.

lead acid, nicd, nimh, nife chems,  a few volts off, ok you heat things up a little, they'll get over it, but lithium.  you are playing with fire.. literally.

aaron

[electrictwowheeler]

they make a bunch of ssr's that would do exactly that.  you could also get a cheap ass controller to look at the voltage and put out a fleapower  signal to drive the ssr, or use standard 4-20 control or 1-5 if you lost the resistor .  this is more crap in it though making things more complex.
you could repurpose a simple ass balancing circuit to be the pilot, steal one out of a bms on a battery thats tits up, hook it across the 2 packs if there is more than 50mv of volt diff it'll be active keeping the ssr open, when the voltage drops in specs it lets the ssr go., .. a few wires and the main battery as the pilot and you are in business.  if you really want to be geeky use the balancer as intended so it slowly brings the batteries equal and when in specs turns around and parallels them properly via the ssr.    how nerdy do you want go get with it?

aaron

How would a solid state relay disconnect the packs if the voltage difference was say more than 2 volts? What exactly is a " cheap ass controller"?
I sort of understand how you could repurpose a balance circuit out of a bms, but how would you get the packs to within 50 mv of each other? If you did, what would the voltage difference be when you give full throttle in sport mode? I expect that the pack voltages differ considerably from each other when the bike is ridden. If the difference in voltage was more than 50 mv, wouldn't the ssr disconnect?
The idea of using a balancer from a bms to bring the packs into balance is interesting, but aren't they designed to work on the individual cells at like 4.2 volts. My understanding is that at a certain voltage, just under the max cell voltage , they connect a small 100ma or so resistor across the cell . How long would it take a 100ma load to drop the voltage of a 15 KWH battery even one volt?  How would you connect such a circuit between the 2 different packs? This is probably all way over my head.

[electrictwowheeler]

That is a great idea. I don't have the expertise or knowledge to build that or answer your question. No doubt that it is possible and would solve the safety issue. The Zero power tanks do that. Maybe someone with the right skills can chime in. I did test the inrush current today with the main pack 2 volts higher than the extender and it was only a peak of 6 amps and then slowly tapered down from there. The extender is only 9% of the combined packs capacity.

I'm not sure which pack would self-discharge first. I would assume it would depend on the health of the cells and what the current draw of the BMS is, or in the case of the bike, any associated electronics that are drawing current while it's turned off.

I should add that the circuit is protected by a 60 amp fuse so there will not be hundreds of amps burning everything up. The BMS is rated at 60 amps continuous, the contactor at 50 amps continuous, the wire has an ampacity of 80 amps and the cells are rated at max 10 amps each discharge so 60 amps.  These are LG M26 cells, 2.6 ah each, 6 in Parallel.
 
There is an issue with cell balancing. My bike has the replacement BBC battery capped at 12.6 KWH. They do that by limiting the charge voltage to a bit over 115v. The extender BMS is set up to charge to 4.2 volts per cell, so 117.6v total. it will have to be fully charged off the bike occasionally to retain balance.

Fuses lag a bit in protection as they are a melt to protect device so high current passage is possible (albeit very briefly).
Time delay fuses extend this time as they allow for inrush current (as apposed to sustained).

Another idea for cross current protection is to install diode(s) to prevent back flow.
To allow on bike charging, wire it/them through the N.C. contact of a double throw contactor(s).
Figure out how to energize the bypass contactor(s) when in charge mode. The N.O. contacts would bypass the diode to allow back current for charging.

Diodes would not allow for regen. Current needs to flow back and forth between the packs when the bike is ridden. Diodes could protect if the extender was at a lower voltage than the main pack, but, what if the extender was at a higher voltage?

[electrictwowheeler]

As so often happens I tend disagree with some other people who voice their concerns.
If you are worried, use a two stage switch and a headlight bulb (or 10 ohm 10 watt resistor) when connecting the batteries together. First connect them through the bulb/resistor. That gives you current limitations. After few seconds connect them directly. If you want to be fancy you can connect an ampmeter to the first stage and only connect directly once the current is below couple of amps.
I would keep the batteries permanently connected together. Then discharge doesn't matter, the voltage stays equal.
One volt difference is fine, also two or three.  As soon as the contact closes the voltage will become same between packs. Inrush of current will drop fast and be pretty much safe level within few seconds. The batteries Equivalent Series Resistance (ESR) basically does not allow enough current to flow to damage the batteries at these low voltage differences. As long as one battery is not fully charged and the other one is empty.

I can't leave the extender pack permanently connected to the main pack because then when the bike was shut off and the main contactor was open, the extender pack would still be connected to the controller input. This doesn't appear to cause any problems but would slowly discharge the extender pack so that the next time I turned the bike on there would be inrush current from the main pack into the extender. Also, one of the ideas behind the extender pack is that it is removable. I can use it or not, it adds weight and takes up storage space. I will only use it for long rides where chargers are far apart.

What the safe voltage difference would be to connect two battery packs together is going to depend on many variables such as pack size, age, internal resistance, system specs.

It is very easy to get the pack I am using to within a volt of the main pack. That is not a problem. But what if I misread the voltmeter or my brain was just not working that day. We all make mistakes. What if I connected it with a 10 volt difference by mistake? It would be nice to have some protection, but it will probably be impractical to actually make that happen. The best plan will probably be to just have the smallest fuse possible and be careful. I seriously doubt that I could damage the main pack. It would be the small pack that would suffer. How about an automatic ejection system for the extender pack if it catches fire, lol

[T.S. Zarathustra]

a battery that can hold several kilowatts of stored energy, even with a modest 2 or 3 volt difference can push hundreds of amps across the connection, and continue for a while until its stored chemical energy has exhausted into the other battery to bring their volts to being equal in which case the current will slowly stop flowing as the voltages equalize out.  this will severely stress the batteries, the bms and other components.  lithium does not like this, at all.

you have to remember with lithium.  at the top and bottom of the curve, yes a few volts, holds little energy, but lets say from 50 to 80 percent charged, a nice middle range, there's a lot of power in those 2 or 3 volts of drop.  when you hook all those batteries up in series, you have the ability to push some hefty amps, for a while, to equalize it out.  even a 20 second inrush can be enough to pop internal protections or cook stuff that isn't designed for that kind of load over time.  high current can generate high heat real fast too.  if you have a weaker battery, that may cause a hot spot, which is enough to start a slow cook off, it overheats there, it can't get rid of the heat fast enough, starts stewing,  then finally cooks off.

ive seen more than once where a tech will touch something, huge current thru pack for a second or two, he jumps, breaks connection.  you ok??  yah, what the hell dumbass... derp. is the pack ok... it looks ok,  well keep an eye on it...... feel it.... it's a little warm but nothing bad. ok...  5 minutes later the thing starts smoking, the packs are swollen and its growing, ok get that fucker outside now!   a minute later its on fire as the cells sympathetically pop off one by one.

now, can you get away with it, yes, depending on your situation it may not be a huge issue with your setup, but this requires you to have the brains to understand YOUR setup and know it's behavior, and that it is safe to do, but overall, is not a good habit to develop.  The idea of using a light bulb or some sort of current limiting device in line is an excellent idea, that many use all the time.  this lets you see the current flow as well as volts, so you can monitor and after say 5 or 10 minutes when the batteries have equalized, then you can directly connect them w/o a limiter and they'll be fine.

lead acid, nicd, nimh, nife chems,  a few volts off, ok you heat things up a little, they'll get over it, but lithium.  you are playing with fire.. literally.

aaron

Please remember that these are stationary voltages. As soon as even a small load comes on the higher voltage battery its voltage drops. And vice versa for the lower charged battery. As soon as it starts to receive charge its voltage increases. Battery receiving charge has relatively high ESR. So small voltage difference has little effect.

[ESokoloff]

Diodes would not allow for regen. Current needs to flow back and forth between the packs when the bike is ridden. Diodes could protect if the extender was at a lower voltage than the main pack, but, what if the extender was at a higher voltage?

Ok, back to the drawing board……

First how big of a deal would that be if the extender didn’t receive charge during that brief period?
Because the extender is an order of magnitude (1/10) less of the main, it’s my thought that it will typically tend to react faster to levels of charge (mains mass acts as a buffer).
Therefore (if I’m right) it’s the extender that needs protection from the main & not the other way around. 
Second, if counterflow is required/desired during operation, segregate the paths (charge/discharge) with separate diodes with suitable sized fuses to handle an acceptable current for the 4@18650 2.5ah battery arrays. If the fuse size turns out to be the same value on both circuits, then it makes my suggestion of diodes superfluous. 
Maybe use a resettable circuit breaker instead of a onetime fuse???

How does Zero handle this protection on their power tank?

[Specter]

the ssd's have turn on points on them, many you can control when it activatres,  for DC that pretty much is eliminated, but a cheap controller, anything that can monitor voltage and put out a signal, arduino or raspberry would work, or even a simple monitor.  you could get fancy like a yokogawa or something of that nature but that's a lot of money and way too much for just this small purpose. 
you monitor the voltage difference between the packs with whatever you choose to monitor, and that in turn sends the signal to the ssr to turn on or off.
some ssr's have an a and b input and depending on the model, again, you have to do your own homework, will turn on, or off depending on what's on those inputs, or a difference between those inputs. *gee it's almost like they are made for this purpose huh*  you select the one for the job and plug it in. 
i don't know what parts you have or how you want to run it, if you want something else looking at the volts and send a signal to the ssr, that works and gives you more control, or if you want something less sophisticated, like the a-b input which turns the relay on if volts between the points is less than say 0.5. 
i know they make them, not sure how many amps they go up to, you'll have to get one that will fit your project. plug that in and it will only turn on if the batteries are within the set range of each other.  most have a pilot light on the ssr that will tell you if it's activated or not.

plan your rig carefully.
aaron