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Li-Ion battery: Too much voltage?

Discussion in 'Power Electronics' started by Madsalts, Feb 27, 2019.

  1. Madsalts

    Madsalts

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    Sep 25, 2014
    I have a Minn Kota trolling motor. I'd thought that Li-Ion batteries were too expensive, but I see that I can make them out of individual cells for a reasonable cost. The problem is, I think that they put out too much voltage. My trolling motor is straight forward, but I've made a pulse width modulator so that I can run the motor wide open, yet regulate the speed without the resistance that the motor uses to regulate speed.

    The Minn Kota website doesn't recommend using Li-Ion batteries, saying that the voltage is too high. If I remember correctly, I can use 6-90 V at 15 A, so I don't think that this would be a problem. As for the motor, well, who knows.

    A trolling motor customarily uses a deep cycle battery, which has a voltage of about 13. Li-ion cells are either 3.6 or 3.7V, so that I would have a 12V battery of 14.4 and 14.8V respectively.

    Everything is off warranty, so that's not an issue. Think this would be a problem?

    Thanks.
     
  2. Bluejets

    Bluejets

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    Oct 5, 2014
    Lithium ion or LiFePo4 are nominal 3.3v per cell.

    Lipo's are nominal 3.7v.
     
  3. dave9

    dave9

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    Mar 5, 2017
    I suspect that if you have already tested your custom PWM with the intended battery and the motor works as intended, that it will work acceptably with the 14.(n) Li-Ion battery, but that you risk overheating it if you run at full throttle, with the PWM controller, or especially without it since that's one less forward voltage drop.

    I am wondering what you hope to gain. The same capacity in Li-Ion is usually more expensive. If you use cheap generic Li-Ion cells they might have a fake overly high capacity rating and also not be capable of much current. You do realize that their capacity is derated for current and a trolling motor uses enough to do that yes? You may find you need a lot of paralleled series of cells to achieve the combination of enough current and runtime. Plus, then you need a QUITE elaborate and expensive balancing charger for the battery, cannot use the same charger a deep cycle lead acid uses, cannot even use a typical hobby charger for a single series (n)voltage pack because you have paralleled series. It can be done, but it's not cheap to do from scratch.

    How much current does the motor draw? Why do you need the PWM controller? It seems more straightforward to me to use a buck converter to reach a lower motor input voltage (with a significantly higher single series of Li-Ion cells, way above 30V (target battery voltage depending on what products you find to suit the need unless building that from scratch too)) then use the motor's throttle for speed?

    We lack too much information. Your question only scratches the surface of how to do it with Li-Ion if that even makes sense. It is NOT just as simple as wire 4 x 18650 cells in series and call it a day, for example. The project is frankly more complex than a single forum topic is designed for unless it is a builder's type topic where you lay out and show what you did rather than just a voltage question.

    For that matter, you could do away with all this switching supply and just put 2 of 3 diodes in series to drop (shed as heat so heatsink them well) a ~ couple volts off the Li-Ion battery. While not the most efficient, I am generally in favor of not having to do waterproof heatsunk experimental projects to power a boat! I mean when trying to be within a budget since you wrote cost and merely buying the cells is not much difference in cost compared to the rest to do it right/safely. ;) I guess in that respect I'd just say don't do it, deep cycle batteries are not scarce.
     
    Last edited: Feb 28, 2019
  4. Bluejets

    Bluejets

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    Oct 5, 2014
    You could switch cells in and out of circuit to get roughly some speed control.
    Would depend on the extent of the current draw as to whether it would have adverse battery life as a whole.

    Don't understand why you need any further speed regulation though as you said you already have a pwm controller operating.
    I've used similar outboards before and found them to be really good as stock standard when it came to battery life off a regular everyday lead acid car battery.
     
  5. Audioguru

    Audioguru

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    Sep 24, 2016
    An ordinary "3.7V" Lithium cell is 4.2V when fully charged. Then four in series make 16.8V which is too high for your system.
    Previously you were told to throw away the extra voltage with some very hot diodes.
     
  6. Madsalts

    Madsalts

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    Sep 25, 2014
    Thanks for the replies.
    First off, I'll say that I understand that all of this might be impractical. I'm looking at all this in an effort to power a canoe or kayak with a 30 amp (max) motor. I'm interested in going this route so I don't have to pick up a 60lb battery and set it in the boat. As for the PWM, I've used it with standard deep cycle batteries, but not with a lithium bank. Maybe I'm wrong in my understanding of the motor's requirements. I don't want to use a resistive speed control because of the wasted energy. This is why I made the PWM. It's my understanding that the PWM lets the motor run on the full voltage that is available, but only intermittently. It seems that with a battery that has too much voltage for the motor, a PWM alone would be insufficient.

    I like the idea of using a buck converter to bring the voltage down. Perhaps I could even switch between a few, or one with a diode to give myself a choice of several speeds.

    I did not know that current is likely to be derated. Upon further reading, perhaps lithium phosphate would be a better choice. This is a Li-Ion that I was looking at, though:
    https://www.batteryjunction.com/samsung-inr18650-25r.html?
    gclid=EAIaIQobChMIyafvzbbb4AIVg47ICh0DbAkREAQYAyABEgLmYfD_BwE

    I knew right from the start that I'd need a special charger. If a Li-Ion charger would have to be further modified as a result of the configuration of cells that I'd likely have to employ with this design, that this right there would make things too much of a hassle.

    As for obviating the balancing charger, would it be possible to charge several series banks with an off-the-shelf Li-Ion charger, then simply wire them in parallel when putting them on a kayak?

    If all this seems like a bad idea, by all means, I'll take that advice.

    Thanks
     
  7. dave9

    dave9

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    Mar 5, 2017
    Yes an off the shelf hobby charger can do single series of cells one series at a time then wire in parallel later, except you still have the issue of terminating discharge when any cell drops too low, but this is an issue even with a single series of cells. For this reason I would not get unprotected cells like you linked, or else you also need a discharge termination circuit to monitor them all.

    Audioguru made a good point about the full charge 4.2V voltage, it would be quite wasteful with a series of diodes. A buck converter could have variable output voltage to suit your speed control, just limited to the 12.6V of a lead acid battery. As mentioned previously this would allow using a greater # of cells in series, and relax the current requirement per cell.

    A 30A motor is going to need a lot of cells if you want to go anywhere without a paddle.
     
  8. Audioguru

    Audioguru

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    Sep 24, 2016
    I do not know what speed you normally run the motor at and do not know how many Ampere Hours you need for a battery.
    Maybe 50Ah if you average 10A for 5 hours? The little 18650 cells are only 2.5Ah so you would need 50/2.5= 20 banks of four cells in series which is 200 cells. Reducing their voltage and charging that many of them would be a nightmare.

    Safety: If 19 banks are fully charged but the 20th bank accidently never got charged then the current into it when you connect it in parallel with the 19 charged ones would be 20A x 19= 380A or more which would cause a spectacular explosion and fire.
     
  9. Bluejets

    Bluejets

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    Oct 5, 2014
    As I said previously, we used one of these little units to power a 12ft aluminium boat with all our fishing gear, beer, ice, two people and had no bother with using an everyday lead acid battery.
    I think you are being a bit over the top really with any modification.
    The drive system and the propeller (pitch/diameter)seems to be engineered towards fairly high efficiency as standard.
    Actually, first time i was surprised how well it performed.
    Motor certainly did NOT draw 30 Amp under those conditions.

    If you have drag racing in mind perhaps then..........:eek:
     
  10. Madsalts

    Madsalts

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    Sep 25, 2014
    Good observation. I think full power would be more like 15A.

    Maybe lead acid would be better. I have nothing against lead acid per se, I do have something against lifting a 60lb battery in and out of my car. This battery (https://www.amazon.com/VMAX600-Battery-20ah-cycle-ideal/dp/B00896SUHM) as an example, I'm not committed to it, provides 20AH and is pretty light weight, and could be wired in parallel.

    As far as "off the cuff" calculations regarding run time, should I assume that the amp hours listed on batteries are twice of what are actually available? I.e., if I have a 10A motor, should I assume that I can run it for 2.5 hrs, since I will only be draining the battery to 50% capacity? I ask this knowing that there are other factors involved in calculating discharge, such as ambient temperature, rate of discharge, etc.

    Edit: Math explained in following post.
     
    Last edited: Mar 1, 2019
  11. Bluejets

    Bluejets

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    Oct 5, 2014
    Your calculation 20Ah at 10A discharge is incorrect at 2.5 hours.
    I make it 2 hours BUT discharge capacity is wholly dependant of the discharge rate and one has to consult the manufacturer discharge curve to arrive at a close figure. I say "close" as all batteries are slightly different.

    I would NOT be running lead acid battery ( or any other for that matter) in parallel.
    Many do but it is a dangerous practice.

    As for weight, most 40Ah batteries I looked at on Google seem to be around 10Kg, not 60lb as you quoted.
     
  12. Madsalts

    Madsalts

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    2
    Sep 25, 2014
    I should have proofread my post. I was calculating based on the 50 AH mentioned in a previous post, not based on the 20ah battery.

    I'll take your word that it's a dangerous practice to run batteries in parallel, but why? An EE once told me to hook them in parallel a day or two before use so that they could all equilibrate, then run them hooked in parallel. I'm not arguing with you, I appreciate the advice. I'm just wondering why.

    So, it seems that I'd need seperate batteries between which I can switch. To what capacity is it customary to discharge lead acid batteries? I've heard some people say I can discharge them to 50%, and others say that I can discharge them down to 20%. Thanks for pointing out the issue of the discharge rate. I think that "amp hours" by convention are calculated at a 5A/hr discharge, unless otherwise mentioned.

    When a battery is rated for "X amp hours", does X refer to usable amp hours, i.e., the amp hours of bringing the battery to some partial charge, or does x refer to how many amp hours would be available if the battery were drained totally?

    Edit: I looked at a discharge curve for a deep cycle battery comparing discharge rate to current draw. This isn't applicable to other batteries, but it gave me a rough idea of what to expect. It seems that drawing 10A (approximately what I'd draw when going at a reasonable speed, but not full throttle) results in halving the available amp hours.

    So, If I assume that a 20AH battery can be drawn down to 20% capacity, this leaves me with 16AH to work with if I'm drawing 5A. So, if I'm drawing 10A, I have 8AH to work with. 8AH/10A = 0.8H = 48 min of run time. This might even be generous. I also assume 16 lbs/battery.

    So, for a 20AH battery:

    1 battery, 20AH =48 min = 16 lbs
    2 battery, 40AH = 96 min = 32 lbs
    3 battery, 60AH = 144 min = 48 lbs
    4 battery, 80AH = 192 min = 3.2 hours = 64 lbs

    For a 35AH battery (https://www.apexbattery.com/12v-35a...DOXkc3ZqQu922YviOJvBEtcLwvYnyvWBoCuisQAvD_BwE) I'll use the same assumptions as before (but with a mass of 23 lbs)

    1 battery, 35AH = 84 min (1.4 H), 23 lbs
    2 battery, 70AH = 168 min (2.8 H), 46 lbs
    3 battery, 105AH = 252 min (4.2H), 69 lbs
    4 battery, 140AH, 336 min (5.6 H), 92 lbs

    I wonder how accurate this is. Again, I'm really glad you mentioned the effect of discharge rate on AH. The above numbers seem more in line with my experience than they were before I halved them. Lifting 23 lbs at a time is OK, and having 3 batteries would be pretty good, without the hassle of lithium.
     
    Last edited: Mar 1, 2019
  13. Madsalts

    Madsalts

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    Sep 25, 2014
    Out of curiosity, what is your current draw, battery AH, and run time?
     
  14. Audioguru

    Audioguru

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    Sep 24, 2016
    How will you measure 50% capacity? Do you have a circuit that averages the current drawn over time?
    The voltage will be high with no load then it drops way down when loaded so the voltage cannot be used to determine remaining or used capacity.
     
  15. Madsalts

    Madsalts

    73
    2
    Sep 25, 2014
    I was going to use an off the shelf capacity meter that can be connected in series b/t the battery and load. I know that they're not perfectly accurate, but I'm hoping that one will give me a ballpark idea of remaining charge. Perhaps these things are of little value, though.
     
  16. Bluejets

    Bluejets

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    Oct 5, 2014
    No longer have it.
    Battery was a standard 40Ah car lead acid battery.
     
  17. Madsalts

    Madsalts

    73
    2
    Sep 25, 2014
    Well, I think everything's been answered. Thanks everyone, for all the help.
     
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