protect controller during battery switch over

[carbon.nanotube.capacitor]

Hello hello,

I've modified my scooter style ebike (a Kaishan K500WS) to have more tourque at the flip of a rocker switch. It's set up so that the rocker switch directs 12 volts from the dc-dc converter to a relay mounted on the controller, which bypasses half of the two shunt resistors. This results in an impressive increase in torque, but a slight heat build up in the battery connector (a three prong connector used to power computers and other electronic devices). This connector does not get dangerously hot, but it means it is losing energy. Oh well - the torque increase is controlled by a switch for the purpose of only using it when necessary.
This is nothing to do with the reason I began writing this. You see, my next modification will be to wire in a second, smaller set of batteries. these will be switched to with a relay, controlled by a rocker switch. However the relay get's power to do the switching from the dc-dc converter, which will lose power when the relay disconnects the 1st battery. I'm hoping that a capacitor from a dead charger will provide enough power to keep the relay charged until it switches all the way to the 2nd battery and gets more power again. Anyway, the question I have is this: Will the controller be damaged if this momentary loss of power occurs while the motor is under load, from the throttle being turned, especially up a hill?
I plan to avoid this theoretical problem by using a second relay. This relay would normally allow the throttle signal to reach the controller, however when the battery relay switches, so too will the throttle relay, disconnecting the throttle signal to the controller. Then once the throttle relay reaches second position, the throttle will be re-connected to the controller, because both default and switched positions on the relay connect the throttle. It is only during the switch from one battery to another, which will take less than a second, that the throttle is disconnected. this will perform two purposes: It will eliminate the problem of the motor being under load, and therefore the controller will be "at rest" while power is cut. And it will reduce the current flowing through the battery relay, thereby making it last longer.

If my theory is correct, which will be tested Monday, this setup will be "idiot-proof". I'm hoping.
As for the relay on the shunt resistors, it is rated for 30 amps, which should be more than enough, but I suspect it too will last longer if switching is done when the motor is not under load. I suppose (although I doubt it's necessary) a throttle relay could be engaged while the high-torque switch is activated. I won't bother though. Besides, if the relay fails, it's not hard to replace.

Any thoughts would be appreciated. Feel free to point out any mistakes - It's always nice to learn mistakes the easy way. Thanks.

-Colin

 

[Link]

Anyway, the question I have is this: Will the controller be damaged if this momentary loss of power occurs while the motor is under load, from the throttle being turned, especially up a hill?

I'm kinda tired right now (2:39AM), so this is all I really got out of that.

No, the controller will be fine. If you don't believe me, you can check this just by not connecting any batteries and spinning the wheel.

 

[TylerDurden]

Sounds ok at first blush...

As you mentioned, switching batteries under load is not good.

If you put a cap on the coil of the power-relay, you might delay the switch so it breaks after the throttle reaches zero.
If you hack a cap into the controller where the logic circuit taps its power, the controller might not blink.

BTW, You can parallel relays for increased capacity & longer life.

 

[carbon.nanotube.capacitor]

Thank you for confirming my concern Tyler. Parallel relays... Good idea, thank you. In these instances it will not be necessary, thankfully. I like the idea of placing a capacitor on the logic circuit of the controller, however inevitably this is all testing to see if this will be viable for customers so modifying the controller in that way might not be time-effective. Already this is a procedure which I will have to become proficient with before it will be lucrative (breaking even would be nice).
Would a capacitor have to be in series or parallel to delay a relay? Parallel?
In theory, do you think that cutting the throttle signal to 0 will be enough for the controller to be safe? Remember that the switch over of the relay will take less than a second.

Thank you!

 

[TylerDurden]

An R-C across the coil will delay a relay for a bit.

I guess the bigger question is the reason for switching the supply.

Is this a "dork-switch", or something more sophisiticated?

 

[carbon.nanotube.capacitor]

Na. This is just to extend the range of the bike. The range is already quite good - but there are two towns you see: Victoria, where I live, and Sooke, about 45-50kms away by packed gravel trail. (The trail is more ideal than the road, not just to get away from traffic, but because the majority of it is level - it was once used by a rail car in the early 1900's. Is called Galloping Goose). I can ride the bike out to Sooke just fine, having used about 75% of the 20ah battery. Once there I charge up at my mum's. My plan is to try a 12ah battery under the seat to try and get to Sooke and back to Victoria without a recharge. Of course weight will be a consideration, seeing as I'm 155lbs and the second set of batteries (4 12v 12ah) would be around 30lbs. It would be nice if heavier customers would be able to do the same.
Although, if the second set of batteries were 7ah, they wouldn't be as heavy. And one could surely find a donor for some electricity for an hour at a shop or cafe. Ultimately the second battery isn't essential for a return trip to Sooke - but there's an abandoned town called Leech Town the same distance from Victoria, and it would (probably) be fun to go there and back. Since there's no place to recharge there, the second battery would be nice.

Life story short, it's to extend range.

-C

 

[TylerDurden]

The way I understand it, if you're going to carry the weight anyway, it's better to use the energy in parallel to reduce the DoD on both sets. You'll get less Peukert effect and the batteries should last more cycles.

FWIW, you can switch in a make-before-break manner, that adds the second group before subtracting the first. With the same voltage packs, the system would not be perturbed whatsoever. This point is moot, in the case of extending range with the same voltage; as just mentioned.

 

[carbon.nanotube.capacitor]

Wouldn't connecting two battery packs of different amp hour sizes together in parallel be as damaging as having a weak battery in a pack? I realize it would be good to simply have a bigger single pack, but there are a couple of reasons having two packs is better: 1 - the bikes floor (for the feet) is where the 20ah battery is, so it is already the right size and shape for a abttery pack of 20ah. 2 - the extra weight of more batteries will be substantial, so being able to remove the second battery for shorter trips will be beneficial.

Perhaps clarify for me what you meant about connecting more than one battery together?

Thanks,
-C

 

[Link]

Wouldn't connecting two battery packs of different amp hour sizes together in parallel be as damaging as having a weak battery in a pack?

Nah, paralleling up same voltage/different capacity packs is no big, sometimes even between different chemistries. They share the load, reducing stress on the packs, too.

 

[carbon.nanotube.capacitor]

So just to be clear - two batteries of different capacities can be connected in parallel with no consequence. Connecting two batteries of different capacity in series is bad because the higher capacity battery discharges into smaller one? Is this correct? Because this would mean I should wire batteries differently - rather than have a switch toggle between them, I should have no switch and just connect in parallel?
But:
1. What if the small battery is connected when either of the batteries is not fully charged? Say the main battery is half used by the time I get home to pick up the second battery - If I plug in the second battery will there be a problem?
2. How should the batteries be charged - with seperate chargers or with one charger? (I know a single charger will take longer to charge a larger battery, but would the batteries be harmed if they are in parallel while charging)?

 

[Rassy]

c.n.c wrote:

So just to be clear - two batteries of different capacities can be connected in parallel with no consequence

If the current resting voltage of one battery is lower than the other battery, they will equalize as quickly as the connectors and wiring between them will allow. Under some conditions this could cause a problem. When I hook two battery packs in parallel I always check that the voltages are about the same first.

Connecting two batteries of different capacity in series is bad because the higher capacity battery discharges into smaller one?

I don't think this is true. It is bad because the lower capacity battery will be exhausted before the other one, and could thus be damaged. i.e. if one battery is 20AH and the other is 7AH, after using 14AH the larger battery will be sitting at 13AH remaining and the smaller battery will be at 0AH remaining. any further use of this will damage the smaller battery. In this example I am assumming both batteries are of the same voltage. If they are of different voltages their drain rates would vary.

 

[carbon.nanotube.capacitor]

And I should clarify - I'm talking about a 20ah pack of 4 12v batteries in series (48v), so the two leads from this pack would be connected to the two leads from a 48v 12ah pack. Would each battery in the pack have to be connected to each battery in the other pack seperately?

 

[Rassy]

Would each battery in the pack have to be connected to each battery in the other pack seperately?

No. Just connecting the final Positives and Final Negatives together ahead of your controller gives you a parallel 48V battery pack. As long as the voltage of each pack is close, this should work good. It is a good plan to put a fuse (40 amp?) on each pack.

 

[carbon.nanotube.capacitor]

Thank you.

I've been running on the idea that batteries had to be as close as possible to the same capacity no matter how they're connected. And I've just spent so much time connecting two relays and a switch together to swap between battery packs... but what I'll do is add a switch to connect the two battery packs together so I can test both ways of having two battery packs... Ultimately this is all trial and error to learn what will and wont work so that I may offer customers good options. I sure hope connecting the batteries together will work - the time involved in wiring would be zilch compared to all the fangled things I did with those relays!

Tonight I will connect the batteries together into a pack and solder in the new charger plug, and then tomorrow I will zip around town showing off the bike to the Canada day crowds! Weee...

 

[Rassy]

c.n.c., the good part is you can draw more amps with less sag with all those batteries, effectively 48V 32AH. But, it sure is a lot of weight. I took one trip last year with my trike/trailer with 12 of the 12AH bricks, just over 100 pounds of SLA. They were powering 3 controllers and BD36's at 36V so the batteries were hooked up in four parallel strings of three in series to give 36V 48AH.

 

[carbon.nanotube.capacitor]

Thanks for the information. That sure sounds like a heavy load of batteries! In my set up, I was in a rush and didn't check the batteries before I put the 12ah pack together... Then I went out to Sooke. On the trail ride back, at night, in the pitch black, save for my 3 watt Luxeon LED headlight... I found that the capacity of the batteries was dismal... I got about 15km out of them before the bike was getting close to shutting off... so I switched to the main pack that I had used to get out there in the first place, and that brought me back up to speed until I got back to populated areas... Then I used the low speed switch and cruised along at about 20km/hr, switching between batteries as needed. It really uses a lot less power to go more slowly for some reason. The next day I tested the batteries and found that they were all balanced, they were just dead that's all! Imagine new batteries that haven't been tested and checked before being boxed up for us at the shop! We wont be ordering from that battery store anymore.
Anyway, given the performance of the batteries for the first 5km, and given the range over the whole 90km round-trip, I'd say I'm 98% sure that a 12ah pack under the seat will provide a ride to and from Sooke on one charge. Very nice.

However... before I decide on how to connect the second battery - parallel or solo - I must weigh the differences: For one thing having the batteries separate would mean that there would always be a slightly fresher battery to switch to. Also I wouldn't have to worry about what state of charge, and therefore what voltage each battery was at when I connect them. On the other hand it is much simpler to just plug in the second battery and turn on the bike and go, and not have to worry about switching - and especially not have to worry about switching when there's no second battery connected - I blew a relay by turning that relay on when there was no second battery for it to connect to, which meant that it soon ran out of power and switched back to the first battery which turned it back on and so on... The constant switching burned it out. Oops.

I guess I just need to know: If the main battery is at 52 volts, and I plug in the 12ah battery which is at 57 volts, and the batteries are in parallel... will the voltage difference be detrimental? If so, then customers would have to be careful to add the second battery when both are fully charged. Also, while the two batteries are discharging, that is one 20ah and one 12ah in parallel, would the 12ah run out of energy first and sulfate because of the deep discharge, or would the charge from the 20ah battery maintain it?

Please and thank you.

 

[Link]

I guess I just need to know: If the main battery is at 52 volts, and I plug in the 12ah battery which is at 57 volts, and the batteries are in parallel... will the voltage difference be detrimental? If so, then customers would have to be careful to add the second battery when both are fully charged. Also, while the two batteries are discharging, that is one 20ah and one 12ah in parallel, would the 12ah run out of energy first and sulfate because of the deep discharge, or would the charge from the 20ah battery maintain it?

Hmm, that's a fairly significant difference. I wouldn't try it, personally. However, putting a big diode on each string should solve that problem, albeit at the cost of a volt or two.

The current will be split between the two packs. And assuming that their internal resistance is roughly proportionate to their capacity, the 20Ah pack will be delivering more power than the 12Ah, so they both should be able to deliver power until they're both dead.