Batteries in Watersnake Venom

[Above-the-Fold 15]

I bought a Watersnake Venom SXW 44/36 last summer from Sailor's Corner. I also bought two 18ah lithium batteries, as recommended The combination seemed to work well even though the batteries didn't last as long as I'd like.

This season, however, they are not lasting at all.  Yesterday, with a measured charge of 12.9, the one lasted 250 metres to shore and halfway back before it conked out. I am usually running at 2 or 3; not the faster modes.

It's possible I'm just seeing a floating charge (if that is the term) and they need to be more deeply charged. But it took about an hour or more running the diesel to get that, which is not cost effective.

I'm going to try charging the batteries from mains power  today and we'll see what happens.

The manual suggests a bigger battery (40 to 60 ah) in the first place and that might be the way to go. But in the meantime, I'd like to get at least some performance out of what I have.  

Anybody have any experience with the system or thoughts?

 

MS

[Black Panther 1,587]

No, but you made me Google waterside venom.

[Above-the-Fold 15]

Always glad to help expand the world. I suppose I should have mentioned it's an electric outboard that I'm using for a tender motor.

[khayyam 68]

18ah @ 12v = 216 wh. In crude terms, something like 200ml petrol equivalent maybe (depends on engine efficiency obviously). So I'd say that if you're only getting 400m of travel on the dink then you're not getting anything like those figures. As guest says, lfp is very different from lead acid in terms of discharge curves so I'd second the guess that they're far from fully charged. Mains charge seems like the best test.

[CarpeDiem 430]

Can you post the specs on the battery? Photos of all the fine print, any writing, brand, model, etc would be good... 

Also, how are you charging it onboard? Specs on your charger would be helpful too... 

[K4309 258]

I can do about 8nm on 2l with my Yamaha 2hp. That's at about 3.5knts, weighs 12kg, easy to lift with one hand.

But I guess that wasn't the question. My apologies for interrupting.

[Above-the-Fold 15]

Thanks for the comments and thoughts. 

The batteries come with a jack for a 12v outlet; you hook them up with the motor running. Last summer they were charging to about 13.4 that way. But this year, so far, I can't get them above 12.9.

I think the mains charging is doing a better job, although it's slow with my teeny charger. But the one on charge is now up to 13.4 when I put a multimeter on it and still not "fully charged" according to the charger.

We'll see how it goes. I'll post the stats on the batteries if this process doesn't solve the problem.

 

MS

[CarpeDiem 430]

55 minutes ago, Above-the-Fold said:

Thanks for the comments and thoughts. 

The batteries come with a jack for a 12v outlet; you hook them up with the motor running. Last summer they were charging to about 13.4 that way. But this year, so far, I can't get them above 12.9.

I think the mains charging is doing a better job, although it's slow with my teeny charger. But the one on charge is now up to 13.4 when I put a multimeter on it and still not "fully charged" according to the charger.

We'll see how it goes. I'll post the stats on the batteries if this process doesn't solve the problem.

 

MS

Yeah we need more details to be able to assist.

Assuming LFP chemistry:

If you're charging at 13.4v then they actually are not (really) charging.   LFP needs a voltage differential of ~600mV to push li-ions across the electrolyte.  That equates to ~3.4v/cell.  At 13.4v the cell voltage is 3.35v/cell which isn't really enough to charge past around 30% capacity.

Assuming typical lfp specs, with the correct charger, you could fully charge the battery in ~2hours. 

Don't confuse charging voltage with charged/resting voltage.  13.4v is the fully charged resting voltage after charging is complete and the charger shuts down. You could (technically) charge your battery to 16.8v and the next day it will be ~13.4v this is just how LFP chemistry behaves.   Voltage isn't a indicator of SoC, other than indicating you have over 0% remaining - which is kind of useless. 

Any voltage over 13.6v and the battery is charging. If you were to maintain this voltage the battery would eventually over charge and become damaged.

[Frank 157]

Likewise I had to google that product but It was useful because I'm pondering getting an electric OB at some stage too. I do like the name though and  its almost worth the purchase  just for the bragging rights. When I was trying to wrap my head around the van LiFePo battery I found this diagram of the charge profile "helped me with my thinking", as you can see its relatively flat compared to other chemistries, my interpretation was 

Charging: At 13.6V the battery is starting to charge  from near empty (10%)

Discharge: At 13.6V the battery has just started to deliver the useable 80% of its available capacity.

This site here has some very good information 

https://www.solacity.com/how-to-keep-lifepo4-lithium-ion-batteries-happy/

Here are their Take Home Lessons for LiFePo to prolong life 

Take-Home Lessons

 

Now that we know how lithium-ion batteries work, what they like and dislike, and how they ultimately fail, there are some pointers to take away. We have made a little list below. If you are going to do nothing else, please take note of the first two, they have by far the most effect on the overall time you will get to enjoy your lithium-ion battery! Taking heed of the others will help too, to make your battery last even longer.

To sum up, for long and happy LFP battery life, in order of importance, you should be mindful of the following:

1. Keep the battery temperature under 45 Centigrade (under 30C if possible) – This is by far the most important!!
2. Keep charge and discharge currents under 0.5C (0.2C preferred)
3. Keep battery temperature above 0 Centigrade when discharging if possible – This, and everything below, is nowhere near as important as the first two
4. Do not cycle below 10% – 15% SOC unless you really need to
5. Do not float the battery at 100% SOC if possible
6. Do not charge to 100% SOC if you do not need it
7. KISS (Keep It Simple Smart-person) – avoid series-connected lithium batteries, avoid heated batteries unless absolutely needed

 

[Above-the-Fold 15]

Thanks for all the replies, especially the all-you-need-to-know link.

It appears the issue was under-charging. When I put the batteries on my wee 12v charger, connected to mains power, they went up to 13.6 (on the multi-meter), and lasted MUCH longer on the water. The only difficulty was that it took at least 8 hours to get there.

I have now run one of them to the point that it cut out ; I'll put it on the charger and see how long it takes to get back up.

And it remains open whether the supplied charging system, meant to go in a DC outlet with the motor running, will do the trick in a reasonable time.

 

MS

S/V Above the Fold

[khayyam 68]

Good to know. I can imagine that if you run the batteries all the way down, yes, it's going to be hard to get topped up again just from a DC outlet with the motor running. (Unless you motor a lot more than we do!). But if you do 300m to shore and back, and then charge for an hour the next time you motor, that might be enough to get back to where you started.