Hi Mic, 

I have the same outboard and upgraded batteries - two of them actually - so that I can always have capacity. I've tried some of the same things you have - charging on AC in particular was one of the methods I had hope for being faster and more efficient, but alas it was not.

I had considered getting a voltage buck converter to go from 12-24v and see if I could charge it faster at that voltage directly off the DC bank, which it sounds like what you're doing. Would love to hear the results!

My initial testing has gone well. For example, my 915Whr Travel battery charge time from 28% to 64% SOC = 3 hours. I will try to get a more comprehensive picture of recharge times during this cruising season and will update here.

A DIY build document, as well as an Excel spreadsheet with a parts list and links to parts that I purchased on Amazon is available at this link:

https://www.dropbox.com/sh/nr36fdvvjg5cf8q/AACFjhGRukjUlUvcanTw2rgba?dl=0

Total cost of the parts is $115 (USD) as of today. Plus tax, and shipping if you're not a Prime member.

This is just my idea of a solution. You are encouraged to modify the design to suit your needs. If you have questions (or comments) please post in this thread and I will try to answer to the best of my ability.

Happy DIY'ing,

Mic

@mgfite98034     Wow, Mic, this is one of the best-documented DIY projects I've ever seen, and with a valuable goal. Thank you!

But I'm probably not the only boater who feels a certain amount of ignorance and fear regarding even moderately large Lithium-ion batteries, so I'm hoping that someone with expertise weighs in on any possible danger to what you're doing. On the other hand, I know that Torqeedo uses 18650 cells to build the Travel batteries because they believe they're the safest for this challenging application. Electric bike batteries of similar size are also usually built of 18650 cells -- though to a much lower standard, I suspect -- and optional high current fast chargers are often available for them.

https://batteryuniversity.com/learn/archive/lithium_ion_safety_concerns

https://www.torqeedo.com/us/en-us/technology-and-environment/battery-technology.html

@mgfite98034 well done! The documentation is superb, and the overall build quality is top notch. I really like the components you used and the safety bits you put in place which many people seem to skip for DIY builds especially for prototyping.

Do you see any heat issues with the DC to DC converter being inside a plastic box? I saw you left room around it in your comments, but I'm surprised it hasn't at least gotten a little warm.  I use these sorts of converters everywhere to get rid of stupid wall warts or other power supplies that require AC power. Almost all of them get warm in some way, depending on the load of course.

I also think it is interesting, but not unusual, that the BMS in the battery is limiting the current to a specific amount. In the many things I've tested that have BMS's, which include a lot of batteries for boats/RVs, computer related batteries, and more, the BMS's inside are always the choke point and can also be good/bad in many ways.

One thing to consider is cycle count and the BMS limits. Most products will allow you to charge them at much higher charge rates, but in doing so, you'll reduce your overall charge or cycle count by sometimes up to 50% because of the higher rate of charge. Battle Born Batteries are like this and they use very similar if not identical cells to the battery you are charging.

Their specs say you can discharge at 100 amps continuously, 200 amps in emergency. So one would assume you can charge at 100 amps - but that's not the case. Even though the BMS will allow you to, charging at that rate reduces the cycle count by 45-50% for the life of the battery. Their recommended charge rate is 50 amps, or 50% of the 100 amp discharge rate.

In many applications, that is still fine - what is the likelihood that you'll discharge the battery 5000 times while you own it? That's a lot of times! Even 2500 is a ton of times - that would essentially be discharging the battery every day for almost 7 years nonstop. But it is still something to consider - the battery manufacturer may not even tell you the rate you can charge at, or may be very cautious with their own chargers for that very reason.

One other question - why did you make the 24v DC lead cable so long? I'd make all of the cables as short as possible to prevent any voltage drop - maybe just based on where you want to plug it in?  What sort of amperage do you see coming out of the 12v bank in general when things are charging? 20 amps?

Hi Ben,

Thank you for the kind words. But I’d have to defer those honors to my inspiration, Steve Mitchell. Who’s articles are so well researched, detailed and well written. And of course yourself, and Ben S, Nigel C and Rod at MHT. Thank you all. 
I’m just an engineer so English is a second language  😏

Yes, please! Let’s have others share their knowledge about this topic. 

I’ll write more thoughts when I’m back on land. 

@ben-ellison

Briefly, my thinking on the safety of this charging method for the Travel battery...    I'm thinking that the battery's internal BMS is set up for this approach (charging from a DC source) since the supplied AC wall charger is a simple OTS AC-DC converter that supplies straight DC at 24V (90W). Also, you can recharge the battery from a simple 12Vdc "cigarette lighter" type plug. In theory the current a cig-lighter plug would supply would only be limited by the plug & fuse. I've used this method a lot over several years on both my previous 530Wh battery and more recently on my new 915Wh battery without a problem. (it's just so sloooooow)

When I first got the 915Wh battery I did some charge acceptance current tests using a lab bench power supply. I varied the charging voltage from 12 to 28 Vdc and recorded the current. What I found was that at 12Vdc the CA was about 3A, at 15Vdc it started to level off at about 5-6A, further increases in voltage didn't increase the current.  Current stayed at 5.9A above about 16Vdc. (Sorry I can't be more specific bc I can't seem to find my notes, so I'm going from memory.) Therefore I'm reasoning that the designers feel that 5.9A is a safe place to clamp the charging current. And 5.9A is what I'm seeing on DIY Super Charger's meter.

@stevemitchell

Thanks for the good questions. Regarding heating due to the DC-DC converter being in a sealed enclosure, I was concerned about that. I thought about doing a thermal analysis, for about a millisecond. But decided to take the easy approach and just build it! In actual use it has not been a problem. When I put a hand on the case I can tell that it is slightly warmer than the ambient air temp, but not unduly so. I was planning to make some temperature measurements of the case and the converter but just haven't gotten around to it. I'll update the documentation when I do. If it were a problem you can add water-resistant vents but I don't think that will be necessary. Use in the tropics might be a different story. And I wouldn't advise using it with direct sun hitting it.

I thought about charge cycle count reduction but in the final analysis I came to the same conclusion as you. The characteristics of raw 18650 cells are known to have a lifetime total charge cycles = 300-500. I rarely ever wait until the battery is fully discharged to plug in, 50% SOC is more typical. And we typically run the motor at about half power, so not hitting it with a rapid discharge rate. If I assume that I might do 30 full charge cycles a year (that would be a lot for me) then a 10 year year battery life should be achievable, assuming a conservative 300 total charge cycles. Even with the (relatively small) increase in charge current and given the limited use the dinghy gets I figure that the battery will still out last my need by several years.

Good catch on the length of the 24V charging cord. I agree with you, shorter is better. At least that's my excuse ;-O

The reason the cord is so long is that I didn't know how long to make it! I want to be able to recharge the battery when it is still mounted on the motor on the dinghy. And I haven't decided where to mount the charger. Once I figure that out I will cut it shorter, but not too short (since I can't seem to find my cable stretcher)  Also, the more I thought about it I'm thinking that the battery's internal charger is probably just a constant current type so that once you supply enough voltage to hit the 5.9A max current limit you're good. So a little voltage drop in the cable probably doesn't matter.  But yes, in general shorter is better!

You'd think that measuring the current draw on the house battery bank would be one of the first things I'd do. But we were so excited to finally be cruising after months of stay-home lock-down I forgot to do it. I'll update the documentation when I get some numbers.

Cheers!

Boat problems and family commitments have prevented me from getting as much time on the water as I would have liked. However, I've finally had a chance to update the DIY Travel Battery Super Charger documents with additional notes, measurements and a small, but important, modification to the schematic. The bottom line is that measurements over a couple of charging cycles have shown that this charger is not appreciably faster than the using supplied AC-DC adapter. It is however more efficient. 

As it turned out, my meter was miscalibrated when I made the initial measurements. The actual maximum charging current is about 3.8A when the input voltage is 24Vdc. (Not 5.9A as I had originally thought)  Tests have shown that the charge rate is +10% SOC per hour. For example, going from 50% to 100% SOC will take 5 hours.

This is still quite a bit faster than direct 12Vdc charging. And a little more efficient than using an inverter to power the supplied AC-DC adapter.

I've updated the documentation posted on DropBox with more details:

https://www.dropbox.com/sh/nr36fdvvjg5cf8q/AACFjhGRukjUlUvcanTw2rgba?dl=0

Posted by: @mgfite98034

@stevemitchell

Thanks for the good questions. Regarding heating due to the DC-DC converter being in a sealed enclosure, I was concerned about that. I thought about doing a thermal analysis, for about a millisecond. But decided to take the easy approach and just build it! In actual use it has not been a problem. When I put a hand on the case I can tell that it is slightly warmer than the ambient air temp, but not unduly so. I was planning to make some temperature measurements of the case and the converter but just haven't gotten around to it. I'll update the documentation when I do. If it were a problem you can add water-resistant vents but I don't think that will be necessary. Use in the tropics might be a different story. And I wouldn't advise using it with direct sun hitting it.

I thought about charge cycle count reduction but in the final analysis I came to the same conclusion as you. The characteristics of raw 18650 cells are known to have a lifetime total charge cycles = 300-500. I rarely ever wait until the battery is fully discharged to plug in, 50% SOC is more typical. And we typically run the motor at about half power, so not hitting it with a rapid discharge rate. If I assume that I might do 30 full charge cycles a year (that would be a lot for me) then a 10 year year battery life should be achievable, assuming a conservative 300 total charge cycles. Even with the (relatively small) increase in charge current and given the limited use the dinghy gets I figure that the battery will still out last my need by several years.

Good catch on the length of the 24V charging cord. I agree with you, shorter is better. At least that's my excuse ;-O

The reason the cord is so long is that I didn't know how long to make it! I want to be able to recharge the battery when it is still mounted on the motor on the dinghy. And I haven't decided where to mount the charger. Once I figure that out I will cut it shorter, but not too short (since I can't seem to find my cable stretcher)  Also, the more I thought about it I'm thinking that the battery's internal charger is probably just a constant current type so that once you supply enough voltage to hit the 5.9A max current limit you're good. So a little voltage drop in the cable probably doesn't matter(I've never had a problem with it, used similar setup when hunting with my Ak 47).  But yes, in general shorter is better!

You'd think that measuring the current draw on the house battery bank would be one of the first things I'd do. But we were so excited to finally be cruising after months of stay-home lock-down I forgot to do it. I'll update the documentation when I get some numbers.

Cheers!

I feel like excessive cable might be an issue later on, so I would adjust the length once you've figured out where you want to mount it.