Measuring remaining battery charge – How

For those of us who use an electric motor on a canoe, once we’re on the water how can one know how much ‘useable’ charge is left before damaging the deep-cycle battery by over-discharging? I have been researching this topic and would appreciate comments/suggestions.

Bottom-line is measuring the battery voltage under load with a high accuracy (0.5%) digital volt meter, has anyone tried this? I’ve been looking for a digital volt meter that I could wire back to the stern but so far no luck. Comments/suggestions? Thanks


From what I’ve read (link below) – “A battery ‘cycle’ … is usually considered to be discharging from 100% to some point not lower than 20%, and then charging back up to 100%. Battery life is directly related to how deep the battery is cycled each time. If a battery is discharged to only 50% each cycle, it will last about twice as long as if it is cycled to 20%. Running the battery down totally flat will have a very negative effect on the lifespan of the battery. See the table below for voltages as related to depth of discharge. “ (

Two things

– Last Updated: Aug-16-14 2:40 AM EST –

First, I admit that I perused the article quickly, but I did not see anything about the need to measure battery voltage under load. I DID see mentioned the need to measure open-circuit voltage, but that's a no-load condition. That's not important to you question, but I wondered where you got the voltage-under-load idea.

Second, what do you mean you couldn't find a digital volt meter that is suitable for this? Does this mean you are looking for an all-weather, "dashboard-mounted" unit? I'd be hesitant to be that fancy, as such specialty items are expensive and it's bound to break sooner or later, but if that's what you want, check out the marine equivalent of an auto-parts company. Otherwise, any number of generic volt meters will work just fine. After all, the unit only has to be used once in a while during your outing, so you should be able to rig it up in a waterproof box (like a Pelican box), which you only open every once in a while while you turn the meter on for a moment to make your voltage check. Then turn it off, shut the box cover, and leave it alone for a while.

The only modification you'll need to make to a generic volt meter is to chop the ends off the standard test leads and solder them to long wires hooked up as you wish.

Measuring remaining battery charge – How
Guideboatguy, thanks for your comments and joining the conversation.

In the link I mentioned (, under the section “Care of your batteries” it mentions “This chart is designed to be used when monitoring a battery under load” and the chart reads Voltage Vs State of charge… 12.6V/100%, 12.5/90%, 12.42/80%, 12.32/70%, 12.2/60%, 12.06/50%, 11.9/40%, 11.75/30%, 11.58/20%, 11.31/10%, 10.5%/0%. While the article pertains to RV batteries it is the most informative I’ve found – suggestions welcome. Indeed the article also raises questions about what percentage load, etc. Nonetheless, for me the bottom-line of what it says is that voltage needs to be measured to better that one-tenth-volt accuracy (i.e. 0.5% accuracy = +/- 0.06V), whereas common digital multi-meters ($20-$30) seem all to have a claimed accuracy of +/- 0.50V…

For context about my interest, at the end of last season my wife and I encountered a very stressful situation that could have avoided with some sort of ‘useful remaining charge’ measurement. On a 14 Ft canoe we use a 30# thrust motor with a 27-class deep-cycle battery that was a few years old. Suffice it to say the voltage dropped so low that my automotive Garmin GPS connected to the battery stopped working…

Thus, my broader inquiry is about how to measure a ‘useful remaining charge’ while on the water (… not in some Lab…) and without big spending? Suggestions welcome.

Having just bought a new Interstate Pro-27M battery (180 RC) my best thought is to indeed use a common multi-meter. Along with my GPS I’ll be taking voltage/speed data at each motor speed setting… and keeping track of how this changes as a function of how long I’ve been on the water (typically at top-speed for sight-seeing)… Then I expect to develop some guidelines for when the battery ages… Note this utilizes my multi-meter’s repeatability for my application, even if the absolute accuracy is off.

For me this is a fun project… Comments/suggestions anyone?

another like battery
Run your first battery down to 50%, at full throttle with all your gear and people aboard and take voltage measurements each 15 minutes- suspect voltage may drop faster as volts are consumed. That should give you a pretty good idea how far/fast you can go ti 50%. This should be a worst case scenario inasmuch as half throttle and shut down time (some battery recovery there I believe) will allow for longer time and distances. Just food for thought…

Measuring remaining battery charge – How
Jeffro, thanks for your suggestion. Indeed once I do the volts-Vs-speed mapping I described above I need to do what you suggest. Continuing to search I’ve just found very interesting results from someone who measured amps-vs-speed for the same MK Endura 30 motor I have ( What jumps out of the page is the gross inefficiency of the top speed setting (#5). When going from setting 4 to 5 his speed increased 25% (from 2.8 to 3.5 mph) BUT the amps jumped 95% (from 15.3 to 29.8).

That been said, all these measurement still don’t answer the question of ‘useful remaining charge’ while on the water (without pricy instruments and higher know-how). Nonetheless, once I get/analyze my data (I’m a numbers- &- charts junkie) I’ll bet some trends will reveal themselves. Then perhaps I can get my battery dealer to evaluate the state of my battery on the way back from an outing when I’ve collected good volt-Vs-speed data (with my $20 insufficiently-accurate miltimeter). Comments/suggestions anyone?

PS- While all this may well appear as ‘overkill’ to some, for me it’s a fun puzzle just-for-the-fun-of-it – sure beats watching TV.

1/100 th of a volt
Pay no mind to that table being published on the web to the 1/100 th of a volt.

Voltage vs. State of Charge is only an approximation.

The correlation is subject to uncertainty introduced by many factors that aren’t discussed in the article you reference.

The temperature of the battery when the measurement is made for example, can shift things. A 1 degree temperature difference will be far more important that a 1/100th volt accurate reading.

Measuring remaining battery charge – H
Thanks Mintjulep. Yeah, the effect of temperature… Plenty of YouTube videos describe using a temp-compensated hydrometer to evaluate a battery but that’s not applicable for sealed batteries….

Back to the basic premise… How can a canoeist using an electric motor determine the remaining useful charge of a deep-cycle battery? (E.g. “I’ve been out for a while… can I go further and still get back in time without damaging the battery by overly discharging…”). Is there a practical way to determine what % useable charge is still available? Typical advice is that a battery getting discharged down to only 50% will list twice as long than going down to 20%, and that going below 20% damages the battery. BUT, how can one readily determine the current state of discharge? Comments/suggestions anyone?

tables used for

estimating battery sizes needed for intended power consumption on for example, a sailboat

Your equipment is already in…

ask the motor supplier where motor power drops off with low voltages.

I’d go for 12.2V for battery life.

use the practical method with your usual routing consumption and the max power trial.

Utube has tutorials for multimeter use age.

You might be over-thinking it.

– Last Updated: Aug-17-14 11:47 PM EST –

As far as how to do it, you've already proposed a method that seems as good as any, by using tables of discharge versus voltage and making the decision about how much discharge is enough. Practice that method a little bit and you'll know what your limits are. As you have already pointed out, the biggest variable of all, in terms of battery drain/distance is the power setting of your motor, so you need to experiment with that aspect before anybody's answers have any value anyway.

You have already gotten twice as many responses as would be normal on this site for questions about electric motors. Most people here don't use motors on canoes and never have. You might try to find a message board frequented by the bass-boat crowd, and see what they say. No one uses their electric motors harder than those guys. However, I suspect most of them don't worry too much about their battery's stage of discharge, especially since they don't need to rely on the electric motor for the trip back, and since buying a new battery once in a while, to them, is a very tiny expense in their grand scheme of things.

I'd always heard that deep-cycle batteries were pretty "tough", as far as abuse goes, and I think that's as much thought as most people put into the problem. Thus, they just use them as dictated by their need and charge them when done. I think if I were in your situation, I'd put more value on my time on the water than on the battery itself and wouldn't fret if its lifespan were cut a little short. If it were really important to not run it down more than I wanted, I'd just use a small gas engine and be done with the problem (far more efficient in terms of weight savings and potential travel distance). My second choice would be to bring a second battery. In that case, the second battery could be smaller, since it would only be used to go straight back to the landing if your voltage checks showed the main battery to be nearing your self-imposed discharge limit.

Annoyed with my inquiry…?
Guideboatguy, it seems you are annoyed by what I consider a proper inquiry seeking to interact with others of similar interests. If it annoys you so much, why do you bother reading & commenting? For your consideration, my wife and I are a senior couple who wish to continue enjoying motorized canoeing as long as we can, therefore the importance of knowing if we have enough battery power for getting back safely and on time.

Indeed you have a point on whether I’m posting on the appropriate forum. However, in your suggestion that I check the bass-boat crowd you immediately clarify they don’t have to worry about battery discharge for their trip back… Huh?

Having appreciated the replies thus far it is sure evident I have not found others with experience in my field of interest. Can anyone suggest a more appropriate forum for my inquiry? So far I have not found a forum dedicated to motorized canoeing…Thanks!


– Last Updated: Aug-18-14 8:43 AM EST –

That's for motorcycles, but I'm sure can be used for any electric motor. Don't remember if it needed a 12V lead or not...

You need distance travelled, how fast/how hard are going to ride from this point on, the Ah put in the battery during the charge, how fast/hard have you been riding to this point, the Ah consumed up to this point on your trip. No other way around it. Too many things to consider, some yet to happen (e.g., how fast you will be going so you can estimate the range you got left).

A 100Ah battery may only give you 60Ah at high load, before its voltage drops below safe. At lower loads, or if you let it recover by just sitting around unused for a few minutes, and then use it again at low loads, you will squeeze considerably more Ah out of it vs. one long trip at a high load. You will probably get the full rated 100Ah out of it this way.

Watch this guy's videos for a good explanation and demonstration:

I ride an electric motorcycle lately and the most reliable measures are the distance travelled and the voltage under load. I have learned what a certain voltage value under load means to my remaining range. Voltage at rest is a good indicator to get you started if you know your battery well, but depending on the quality and type of battery it can be pretty unreliable too. But you have to learn what these mean to your range under varying conditions. So, knowing your speed and a voltmeter under load for your voltage and voltage drop under acceleration would be a good way to get an idea...

Sorry about that

– Last Updated: Aug-18-14 8:53 AM EST –

I'm not annoyed, but you do follow up each reply by re-stating the original question, which made it appear that you were looking for replies to come at a faster rate than what's likely here.

I was sort of thinking out loud about the idea of going to a fishing-boat forum, realizing folks there do have a lot of experience with draining deep-cycle batteries, but then I also realized that there are reasons that few people with those boats would worry much about this particular aspect of the problem. I guess I was saying "maybe give those folks a try too, but here's why you might not want to get your hopes up".

And I figured it couldn't hurt to point out that as complicated as the needed analysis is likely to be, and considering how you will need to keep records for a while to come up with a way of interpreting what you measure, an easy work-around might be a spare battery (sort of like the fuel-reserve valve originally put on motorcycles because they lacked fuel gauges).

Gap between paddling and fish forums
Guideboatguy: I know that you know your stuff, and I enjoy your posts, but for those interested in electric motors on a canoe, there is a bit of a gap between the paddling boards and the bass boat crowd. I understand the frustration sometimes of dealing with others in loud motors while paddling, and how few areas are closed to motors. I also want to continue my lifelong enthusiasm for canoes and canoeing, but getting older, and taking children along, caused me to look into getting an electric motor.

Plenty of guys use trolling motors on canoes for fishing, but, trolling motors are designed for planing (high speed) hulls and not for displacement hulls like canoes and kayaks. Trolling motors are thus wildly inefficient, and crazy heavy so as to possibly destabilize the boat. They are relatively inexpensive, but not a good fit for me with high performance, lightweight canoes. I found very little information online about practical electric motors designed for canoes.

At the risk of veering totally off-topic, let me tell you that I took a leap of faith and invested in a Propele Electric Paddle, using brushless motor technology and Nickel Hydride NiMH batteries. Eight pounds for the motor, and 8 pounds for the battery. It is quiet. You can paddle along with it. It is simple and easy to use. For me, this is a game changer. It has allowed me greater range and a margin of safety.

I recently posted a review of it here on pnet. I would like to be able to discuss it on the pnet forums, but I know many paddlers are somewhat dismissive of the idea of using an electric motor. As the technology emerges, I hope this will change.

Not so sure about design & planing hulls

– Last Updated: Aug-18-14 11:55 AM EST –

I can't comment on the motor you chose because I don't know anything about it at all. I have no argument about people using electric motors, and if a person comes to rely on that as they get older, that makes sense. I'm starting to realize that I won't be young and fit forever too.

Though I agree with you that trolling motors (along with the battery) are awfully heavy, for example, compared to small gas outboards, I'm not so sure about trolling motors being specifically designed to propel planing hulls (at displacement speeds, of course). It's true that as displacement hulls go, fishing boats are not nearly as efficient as boats designed to move that way. Though there may be room for design improvement related to pushing these lighter, more efficient boats, I wonder how much so, since a trolling motor runs virtually the same RPM regardless of what size boat it pushes. A trolling motor prop slips a lot under all conditions, regardless of boat speed, so actual thrust is just about the same as stalled-speed thrust. What it comes down to is that a trolling motor designed to push a small fishing boat will simply push a canoe a lot faster, both because it's a lighter boat and because it moves through the water more efficiently at such speeds.

I'll read about the motor of your choice later, to see if anything is said about what they did to improve the efficiency when pushing smaller, lighter boats. Maybe that will be useful info for the original poster too.

Okay, took a quick look at that, and see that a slower-speed, "better-grip" prop is used, which makes sense for light loads. Still, if I needed a motor, I'd go with gas. Right now even my average "after work" paddling trip greatly exceeds the distance this motor can do, let alone regular paddling trips, especially considering the top speed of approximately 2.5 to 3.5 mph drains the battery in less than two hours (even 3.5 mph, which presumably is the fastest speed the motor will go with the lightest boat they've ever tried, is less than my average paddling speed). You'd think they could do something to improve that. Maybe one should buy multiple batteries.

Mostly a matter of efficiency, I suppose

– Last Updated: Aug-18-14 2:52 PM EST –

Just as I expected, a reasoned and technical response. I thank you for it. The makers states: “The Electric Paddle system is so much lighter because it's geared properly for small boats. A trolling motor on a small boat is like a powerful car engine stuck in first gear; no matter how much power the engine can produce - you still go slowly.”

Your ideas on trolling motors sounds about right but I am not knowledgeable or technically-oriented enough to really engage you on it. I suppose it is mostly a matter of efficiency. The Electric Paddle is a variable speed motor.

The prop blades are very different designs. The blades on trolling motors are thick and short compared to the Electric Paddle prop blades, which are long, narrow and made of carbon fiber. I believe the design traces back to today’s high performance, motorized model airplanes.

I would appreciate any other thoughts you may have.

Added: I am usually running the EP at half speed (which significantly increases range in this type of battery (NiMH), according to Propele), doing roughly 2 mph with motor alone. I/we paddle along with it much of the time. I get about four hours running time at half speed, but, obviously, it depends on wind, tides, load, etc.

I have two batteries. At half speed, the noise is a barely noticeable hum. I can see and do more, and I like the back up of being able to return to the put in faster if needs be, and to return easily even if pooped, my elbow tendonitis is acting up, and the like.

Agree that you’re over thinking things
There is so much half-information about batteries on the web.

Batteries have a useful life. That’s it.

Batteries degrade. Every time you use them. Every time you charge them.

Nothing magic happens at 20% of full charge that changes the physics of chemistry of what is going on inside the battery.

Rate of discharge, rate of charge, temperature and other factors all come into play.

Just go figure out a typical use profile for you and buy an appropriately sized battery. Plan on replacing it every few years and go out and enjoy yourself.

I’m no expert

– Last Updated: Aug-18-14 1:53 PM EST –

My comment on trolling motor speed being the same no matter what the load is (the result of prop slippage), is based on what I've seen with the trolling motor on my own fishing boat (though it's a mighty old motor - new ones are a lot better) and a couple of others. The pitch of the "hum", which is an indicator of motor speed, is the same when the motor is first switched on while the boat is stationary, as it is once the boat is up to speed. Rotating the motor into a reverse heading while traveling forward slows the speed down a little, but that's an extreme situation, and the change in motor speed is still pretty small, which indicates lots of prop slippage. Slippage doesn't by itself mean poor efficiency though, as thrust is related to the rate at which water is set into motion (in the proper direction) by the prop (Carldelo, who posts here now and then, is a real expert on such things. I only have a feel for some of the basics).

Years ago, Mercury introduced a trolling motor with a very large, slow-turning propeller. It was supposed to be more efficient for moving a heavy boat for the same reason that tugboats have huge, slow propellers, but I don't think the idea really caught on (actually, I understand that nowadays a lot of tugboats have "propellers" that look more like a pair of egg-beaters extending vertically down beneath the hull, and by means of variable pitch they can produce thrust in any direction, but I think the "big slow propeller" idea still applies in terms of how they are designed).

Facinating area
A fascinating area of interest that I will have to learn more about. I messed around in all types of motor boats as a kid, but as an adult, not so much. Cheers