By now, the process of creating custom lithium-ion battery packs is well-known enough to be within the reach of most makers. But it’s not a path without hazard, and mistakes with battery protection and management can be costly. Happily for those who are apprehensive on the battery front there’s a solution courtesy of a group of engineering students from the University of Pittsburgh. Their project was to convert a pedal bicycle to electric assisted power, and in doing so they didn’t make their own pack but instead used off-the-shelf 40V Ryobi power tool packs.
The bike conversion is relatively conventional with the crank replaced by a crank and motor assembly, and a pair of the Ryobi packs in 3D-printed holders on the frame. The value in this is in its reminder that these packs have evolved to the point at which they make a viable alternative to a much more expensive bike-specific pack, and that their inclusion of all the balancing and protection circuitry make them also a much safer option than building your own pack. The benefits of this are immense as they bring a good-quality conversion within reach of many more bicycle owners, with all parts being only a simple online order away. Take a look at the video below the break for more details.
Those Ryobi cells certainly seem to have carved themselves a niche in our community!
Nice post, thanks :-)
Have few older Ryobi packs with that extended 3 pin comparatively awkward charger connect unlike the flatter one shown here. Quite a few of them were discarded due to it seems batteries not charging or discharging equally regarding the settled voltage. So opened the packs up, drilled holes adjacent to + & – of each battery so external circuits can control charging far more precisely. Only hobby stuff but, gets around many issues.
It’s interesting as there seem odd failure modes – in a couple of cases can be “rejuvenated” to within about 90% of new as if some organic like healing process at work, weird. In one case of several dozen all the cells failed high resistance ie meg ohms, wonder if these cells have a built in eutectic metal fuse, they don’t seem any different to others, maybe experiment fell into part production. The characteristics of all 5 cells in that pack identical high Z, bizarre as all other packs I’ve checked had variations, someone strange one pack same failure mode in all cells – maybe it’s a take – might cut one open. For last 2 months left it in parallel cell by cell with 5 new cells fully charged, we will see :-o)
The Ryobi 18v/OnePlus format was originally meant for a NiMH pack which might explain why it doesn’t have balance terminals apparently? Or is it using DC-DC conversion to work and you’ve bypassed the built in charging circuit?
In all the Ryobi packs I have some with 5 cells in series, others with 2 cells in parallel then 5 of those in series there’s no DC to DC converter, and no individual power transistor type comparator circuit across each cell either. The ten cell packs have a lot of grunt though I have swapped out a few cells now and then And use a low resistivity graphite type glue With pressing the old tabs tightly on the cells (don’t have a spot welder & don’t solder), seems fine for a cordless drill of occasional use – not for eBike sustained of course.
Drilling holes through the sides of the backs directly to the cells ends works well with spring probes to a CVCC industrial HP supply, though I balance manually at present then top up series as intended in the commercial charger then check once more – is fine for time being as not an often used tool.
Ryobi 18v lithiums have a built-in BMS that does balancing and protection
Ridgid also does balancing and protection in-pack
Apparently DeWalt packs don’t have undervoltage protection, and rely on the tool to do it according to some sources I’ve seen. Or was it Milwaukee? Ridgid and Ryobi do have undervoltage cutoff in the pack.
The Ridgid 18V batteries look to be very close in design to the NiCD batteries for an old Harbor Freight reciprocating saw I have.
Ryobi ONE+ was originally NiCd. They went directly to LiIon, skipping over NiMH. It’s quite nice how they integrated over-discharge prevention into the batteries so they could make them backwards compatible.
They’re doing a deal now at Home Depot where you buy a pair of 4AH batteries with charger for $99 and you choose one tool from a selection for free. I got a jigsaw, hedge trimmer, and 8″ pruning chainsaw. I’m all set for batteries. The hedge trimmer is most expensive at $99.
The other nice thing about the ONE+ system is that with a few exceptions, ALL batteries with with ALL tools; the exceptions are several of the high-draw tools that like the One+HP battery varients; I have a palm router and circular saw and those run best with those.
I also like that there are a bunch of ‘niche’ tools that no one else seems to have, like a glue gun that you should get during the promotion, because it’s That Good. (seriously- a hot glue gun without an 18″ cord and no easy way to set it down? YES PLEASE.) They also have a few kind of silly things, like a floating bluetooth speaker with LEDs for a pool (which works pretty well), and a fan with a mister system built into it that’s designed to be parked on top of a 5 gallon bucket (or hooked up to a garden hose).
I’m still waffling on their ‘no gasoline’ ‘generator’ which is an 1800 watt inverter and battery management for four of the 40 volt batteries; the unit also has a sequential charger built in to top up the batteries when grid power is available. Part of the waffling is that with a set of 6AH batteries, it runs a shade under a thousand US pesos, which is a bit more than their own gasoline powered generator of equivlent size…
I bought a ryobi power station & 4 × 6ah 36v batteries for free camping & back on the road & then have it charge as we travel along to our next destination. I provides single use of microwave, toaster & low wattage electric kettle, no noisy generators operating. Petrol generators are obsolete for overnight use.
The purpose behind the “awkward” connection “post” was actually due to the use of NiCd cells when the One+ packs were originally designed. To get 18V from 1.2v cells, you need 15 of them. There are 14 cells in the bottom, arranged nice and neat, and the odd cell is inside that connection post. Because the whole marketing strategy for One+ relies on total compatibility, that post had to stay even when they switched to Lithium cells and utilized less, and smaller cells to achieve the same voltage. The larger lithium batteries just have multiple “packs” of the cells in parallel to multiply the available amp-hours. The third “prong” in the post was for the temperature probe embedded between the NiCd cells to tell the charger what the pack is doing. It is unused in lithium packs since there is BMS (Battery Management System) circuitry built right into the packs now.
There is a company here in the UK selling electric conversions for Brompton folding bicycles that can use a 36V Bosch power tool battery. https://www.nanoelectricbikes.co.uk/ .The battery fits in an adapted bag in a 3D-printed socket, so you can either unclip the bag or take the battery out and charge.
(Ironically though I have several different bags and didn’t want the weight and space of a battery inside, so I ended up buying a 36V water bottle-sized battery from eBay which I managed to mount on the bike frame without fouling the folding mechanism of the bike. )
Rich rebuilds used drill batteries for this micro-car. The USP is you could “supercharge” buy buying new batteries at Home Depot!
https://www.youtube.com/watch?v=erMhK4Hf16E
Got one of those myself. Converting it to use a dual motor configuration.
>mistakes with battery protection and management can be costly. Happily for those who are apprehensive on the battery front there’s a solution
This is not it – there are caveats:
Don’t do the Y harness. If you plug in two batteries that are at different states of charge in parallel, you will break things. Very high currents may flow between the packs and weld the connectors, melt the wires, and overheat the batteries – or at the very least trip the safety fuses and render the power pack useless. It’s safer to simply swap the pack when one runs out.
Each battery in parallel should have a blocking diode or a blocking MOSFET to prevent back-charging by the other batteries. This however will prevent regenerative braking and requires adding capacitors or else the ESC will be damaged by high back-EMF from the motors with nowhere to put the current. Handling multiple cells in series/parallel configurations safely requires a proper battery management system – you can’t just yank a bunch of cells out and charge them without balancing with all the others.
This is btw. the reason why all sorts of “modular swappable battery” schemes for electric cars have failed to take wing. When you plug a bunch of battery modules on the same power bus, the ones with the highest voltage will provide all the power until they are just as empty as the others – unless you add DC/DC converters to each module to equalize the outputs – which means you’re either killing the modules by overcurrent, or paying a heavy penalty in cost and efficiency for the power converters.
I’ve got a bosch ebike system with dual batteries and it does a sort of sequential discharge to avoid this. It enables one battery’s output then discharges it from 100 to 95%. It then disables the first battery and enables the second to do the same 100% to 95% discharge on that one. It then cycles back and forth draining each by 5% at a time. If you plug in two packs with differing voltages the higher one gets used until it’s at the same level as the other. Feels like overkill but it’s better than leaving the packs to figure it out themselves.
It’s smarter that way because again the two packs are not made perfectly equal in terms of internal resistance or capacity (they may be of different age and use etc.), so the drain on the batteries will not be perfectly equal, so whenever you let go of the throttle and the voltage bounces back up, one pack will charge the other for the difference. This internal cycling will waste energy and heat up the batteries needlessly.
Of course, if you insist, you would need to buy a hundred power packs and test each one to find a pair that was as identical as possible in age, use, capacity, and resistance. Then you could simply connect them in parallel.
Also, when you’re talking about multiple amps of current, tens of amps to drive a 200-500 W motor, the length of wiring between the packs starts to affect how much current is draw out from which pack because wires have resistance. As always, the devil is in the details.
The internal resistance and capacity don’t matter much for parallel cells unless you’re running them _very_ hard. Even if you have really badly matched cell capacities, the voltage at the outputs is tied together so the cells can never get too badly imbalanced. As you describe it, the “one pack charge the other” happens the entire time the discharge is occurring too, so the only difference that can occur is voltage drop across the cell impedance, which is pretty tiny on most lithium cells, and as soon as you ease up on the throttle, they’ll charge that tiny difference with very little loss.
Where problems happen is when people try and put packs in series. In that case, differences in capacity can result in over-discharging the cells in the smaller pack and/or reverse charging them if the pack doesn’t have a bms.
If the pack _does_ have a BMS, you’re likely to destroy it, since the fet will typically be rated not too much above the pack voltage to have the lowest on-resistance.
For example, a 6S pack is 25.2 volts at full charge. Its protection fet is likely to be rated for 30v. When you put two packs in series, and run them down until one of them hits LVC, its fet shuts off, but the rest of the circuit is still complete, so you might have 18v remaining in the dead pack when it cuts off (3v/cell), and still 21.5 in the other nearly empty pack. those are in series, and connected through the motor controller, so that 30v fet now sees 39.5v across it.
I did this with my makita batteries earlier this year.
When i plug my electric bike into one of my 18v lithium batteries, with a Ryobi inverter.
I get an extra 5 miles out of it, On the hills of Tennessee.
Doesn’t make sense. Tool batteries are usually priced at a stiff premium when all they are is a bunch of 18650s with a minimal power circuit on top of them. Perhaps less so for Ryobi, an entry-level brand compared to Milwaukee, Dewalt or Bosch, but certainly higher than the actual eBike battery packs themselves.
The smart bit is the charger, not the battery. You buy the charger so you don’t have to design and build it yourself.
However, as you say, there isn’t actually anything really smart in these things – most battery packs are just a bunch of cells in a plastic box with some minimal safety fuses. We would hope that the cells were balanced and equalized at the factory so they would last a long time in use, but they are probably not, because then they get to sell you a new pack sooner than later.
Ryobi actually seems to have a bit more smarts in the battery, but even so they don’t appear to do any active balancing on the cells, so some owners have resorted to opening the battery pack and charging all the cells to the same voltage manually. Otherwise, as they’re charged in series, they drift apart after a while as some cells leak more charge than the others, and the pack starts to appear “dead” even though the cells are actually at different SoC.
Point being, if there is no active balancing BMS, then the first cell to reach full determines when the battery is “full”, and the first cell to reach empty determines when the battery is “empty”. Most dumb battery packs simply don’t charge the cells to very low or high voltages, so they can be hundreds of millivolts out of balance before anything dramatic starts to happen. If this was not done, then the drifting SoC would cause the battery to go up in flames when you attempt to charge it. Of course this means wasted capacity – paying more per amp-hour.
The very simplest BMS is to put a zener diode across each cell, so it bleeds the extra charge to the next cell in line, so you can then “trickle charge” the battery to equalize it, but the zener will leak current (it doesn’t have a sharp cut-off) and the battery will go empty on the shelf.
This is wrong. All of the tool battery packs I have opened have a built in BMS to balance the cells and prevent overcharge and over discharge. The charger is just a dumb power supply. (The BMS is a common point of failure.
Anecdote is not evidence. A lithium battery can be charged with a “dumb” CC-CV charger without any BMS as long as you assume the cells are identical and balanced, and you use conservative charge limits.
While most have the over-voltage breaker in the battery, some brands implement the low-voltage protection in the tool instead of the pack, and the cheapest brands often have no balancing whatsoever.
You should never charge series connected lithium batteries without a BMS. Even if they are perfectly identical and balanced, they will eventually get out of balance and you will end up damaging the batteries (or fire, etc.)
I’ve never seen a lithium battery pack without a BMS and if you come across some cheap one; do not use it. It’s a disaster waiting to happen.
It makes sense for someone like me. I have over a dozen high capacity Milwaukee batteries and 3 chargers already. If there were an easy to source adapter for use with an electric bike it would make conversion pretty cost effective for me.
Check eBay and Amazon for adapters. Milwaukee makes a few different battery shapes, but I see adapters on there for 10-20 bucks for M12 and M18. I 3D printed an adapter for my cousin’s M18 packs to power his fish finder and he loves it.
Yeah, using tool batteries for not tools doesn’t make sense if you need to buy batteries. For me, I already have like 30Ah of Ryobi batteries around, so I 3D print adapters for them to do all sorts of stuff. Kid’s Hot Wheels, portable RC battery charger, portable lab power supply, and USB-PD source. I paid for those mini fuel tanks, and I’m going to use them!!
When I’ve been comparing prices, cheap brand tool batteries are actually not very expensive compared to cheap brand ebike batteries or plain cells. But I still went with doing custom LiFePO4 battery myself, just in case it would be a bit safer.
The draw for me is that I already own those batteries for the chainsaw and other purposes, so no matter how low the cost, the entire cost of buying another kind of ebike battery is totally unnecessary, I would LOVE to use my Ryobi batteries for my ebike. I wish someone would sell this kit as this is a great idea but I don’t have time to build it
I have done a ton projects with Ryobi batteries ( https://www.youtube.com/playlist?list=PLyhwVz7fjRxWD_g7tFK5YP6Y-0aLGJrNi ). I’m not sure how many watts the 40V 4Ah batteries BMS’s are rated for. They should be good for at least 300W since Ryobi sells a 300W inverter for them. So it is good that you used two batteries. Just be careful that you only put in two similarly sized and charged batteries (or add blocking diodes to your design).
Now if we could figure out the protocol to put the 40V batteries into charge mode (with the 18V batteries you can put a 1200W resistor between the T1 and + terminals).
I’m glad that you plugged yourself here, I subbed! Looking forwards to digging in.
“Our system involves two batteries, connected to the motor in parallel via a Y-harness.”
That is going to be a ‘no’ from me, dawg. I mean, I do that type of stuff all the time with my RC batteries, but I’m always super aware of the risk and I do it in a safe environment. It is kind of a “do as I say, not as I do” sort of thing IMO. Paralleling together 2 tool batteries like this with no regard to SOC or cell health is just asking for trouble. Not to mention the BMS’s possible shutting down if they see reverse current while not connected to an actual charger. Also, the batteries are “connected to the motor”? Who wrote this, the marketing guy?? lol
If you aren’t the kind of person very comfortable with lithium batteries, and aren’t prepared to be very diligent with their use, please please please never connect batteries in parallel.
IMO the better solution here, if still trying to keep it with the ‘use people’s tool batteries they already have’ mentality is to use two of the much more popular 5S (18-20V) batteries in series. Most popular brands, have built in power electronics to turn themselves off in the event of an overheat or complete discharge, alleviating the risk of over discharging one of the packs. Unfortunately, 4Ah is kind of the sweet spot for 20V tools, which is a little small in the ebike world. I’ve got a 9Ah I run my shopvac off, so I’d either need to carry another set of 4Ah packs with me to get into the 8-12Ah range. Or buy another 9Ah pack, which at $160, I’d rather just go buy a $260 dedicated 36V 10Ah ebike pack.
Sorry I misspoke. My brand of choice (Ryobi) does indeed have the over discharge protection in the battery, but other brands, like Dewalt, put that control in the tool. I dunno why I forgot about that, it is one of the reasons I like Ryobi so much… So, you know, like make sure you understand how your batteries work.
Having the BMS in the battery pack itself was essential for the One+ system to be completely backward compatible with every One+ tool ever made (and forward compatible with future tools). Otherwise you could not use lithium packs with your older tools that came out in the NiCd era.
Unless you add a diode reversed across your series pack, you’ll likely blow up the BMS when one of them hits low voltage cutoff, and it tries to disconnect with near twice the voltage that it was designed for.
Can you say more about this? I’ve got a Schwinn Stealth electric scooter that normally runs off 3 12v 10ah SLA batteries and would love to convert it by putting two of my Ryobi 9ah 18v batteries in series. I imagine the range would far exceed what I can get from even brand new SLAs but obviously don’t want to ruin the Ryobi batteries, which I use for my bevy of Ryobi tools.
I did this a few years ago with 40v tool batteries for a 36v bike motor. Works great.
Can anyone help me modify the files for a Trek 7.2FX commuter bike? I have no idea what I am doing but I do have access to a 3-D printer through our public library.
Thank you!
Look it up on YouTube.
The EGO Lawnmower battery works well for an eBike conversion. It is 54 volts. Thingverse has a free CAD 3D printer model for the battery holder/controller mount. Works great for my converted Dahon folding bike.