LiFePO4 For Propulsion: Bottom Balancing, Cabling?, Mastech Power Supply as Charger?

[Wireless1]

I have searched diligently in the hugely overgrown LiFePO4 thread for those using them as house batteries, but since I am using these batteries for propulsion, I could not find anything specific to my several questions here. I have 32 Voltronix 3.6V 160AH prismatic cells. I am using them in a 2P16S configuration as a 48 volt battery pack to run my 10.5 Kw PMAC propulsion motor with it's Sevcon Gen4 controller.

Here's the problem; the only way I can fit 32 cells in my boat is to separate half the battery pack and locate it about 3 feet from the other half of the battery in a separate locker. So each half of the total battery will be in a 2P8S configuration. I would like to connect these two halves together using 4/O cables for minimal voltage drop with each lead being the exact same length. (I have the Thomas & Betts pro battery cable crimper). Since I plan to bottom balance the cells at 2.6 volts, I would like to know if anyone thinks this slight voltage drop between battery halves when under load will be of any concern other than perhaps a slight loss of efficiency? I am hoping that in real world use this will not be much of a big deal as long as I don't go over HVC or under LVC. There appears to be a lot guys in the EV car field that are locating their prismatic cells in strange configurations and I haven't found any arguments one way or the other about strapping batteries in unevenly distributions. However, I have found out one thing for sure. The EV car guys really torture their LiFePO4 battery packs. They seem to often go way below LVC just to get it back to the garage! Then they pound the amps back in with one of those huge Manzanita $3,000 chargers at 3C and carry on! Amazing what these cells will take...it really is quite unbelievable.

Lastly; does anyone have any experience using a Mastech Power Supply as a battery charger of LiFePO4? Or any other DC supply for that matter? As long as a DC supply has sufficient current capabilities, and has over voltage and over current protection, I think it would qualify. I know Mainsail has posted numerous times he is using a smaller Mastech supply for individual cell balancing, and 12 volt pack balancing, but I'm am thinking of using the larger Mastech HY6020EX as not only a cell balancer when needed, but also the full on charger to get my 57.6 volts at 20 amps. (limiting 3.6 volt HVC per cell). Since I don't need to recharge quickly I am not willing to invest in one of the huge and fast LiFePO4 chargers. Overnight charging will be just fine for me. Thanks in advance for any thoughts and opinions.

[deckofficer]

I'll be curious what the pros have to say. When you said 3.6 volt cells, that is what caught my attention. The 3.7 volt cells aren't LiFePO4, but after checking your Voltronix cells they are 3.2 volts nominal. 16 cells is the right cell count for a 48 volt system for LiFePO4, 14 cells for LiPO. Is there a reason you don't just want (2) 16 cell strings that you could put on a battery switch? Are you going to run a 48 volt inverter?

[Wireless1]

Hi Bob! Been a while... Our last chat some time ago was talking about your T-Bucket and my Exige. Fast cars! Ha! Anyway, thanks for catching my typo. Yes, you are correct. My Voltronix are 3.2 Volts nominal at 160AH. I guess I had my 3.6 volt HVC embedded in my brain when I was typing. Now it looks as though I can no longer edit my post to correct it to 3.2. Oh well... So, yeah...they are 3.2 nominal. I am going to be bottom balancing this pack. Their data sheet doesn't seem to make much sense regarding their published voltage range and what their graphed range shows, so I'm going to have to be ultra conservative in the beginning and do a lot of amp hour counting and voltage monitoring I guess.

To answer your questions;
Yes, this is a 48 volt isolated stand alone propulsion battery pack. I am not going to use these LiFePO4's for anything but propulsion---no AC inverters, DC to DC converters, etc. My 12 volt house system is still LA and working fine with my solar panels so I'm in no hurry to change over to LiFePO on the house bank...........yet. ;-)

I know I could run my LiFePO's as two separate 16 cell strings and install a selector switch. However, as I drained down one pack and then have to switch to the other, it's highly unlikely they would both end up at a similar discharged state. So when it came time to charge them I would end up having to charge them separately, otherwise the first cell to hit the HVC cut off would shut off the charger and the entire lower voltage pack would not reach as high, or equal a charge as the pack that triggered HVC. So I'd like to keep all the cells as one 48 volt pack.

Have you ever heard any mention of physically dividing a full pack in half like I am considering?

[smac999]

I don't have any experance with lifepo4 but 3' with regular batteries is pretty small. I've seen banks 10-20' apart have issues.

depending on the current draw of your system (no idea) you can always run bigger cable. I have run double 4/0 before on a house bank. but it had 500a of charging on a 1600ah agm bank.

[deckofficer]

The EV group doesn't have any issues with 1/2 the series string some distance away like the Porsche 914 conversions. Also, since you mentioned you still have lead for the house bank, keeping the propulsion bank as a single series string (2P16S) will make the update to "no more lead" much easier.

I use LiFePO4 for my propulsion on my electric kayak. I have (2) banks, one for outbound and the other for return.

[Wireless1]

Originally Posted by deckofficer

The EV group doesn't have any issues with 1/2 the series string some distance away like the Porsche 914 conversions. Also, since you mentioned you still have lead for the house bank, keeping the propulsion bank as a single series string (2P16S) will make the update to "no more lead" much easier.

I use LiFePO4 for my propulsion on my electric kayak. I have (2) banks, one for outbound and the other for return.

Thanks for confirming what I see the EV guys doing. I think I'm going to be fine separating the pack. BTW, I am curious; since amp hours are amp hours, what is your reason for using separate battery packs for outgoing and returning in your kayak? Would not putting them all in parallel amount to the same amount of amp hours available? If there is a strong reason for doing this I'd really like to hear it because this directly relates to what I am doing. If I made these 32 cells into separate 48 volt packs, then one pack could be outbound, and one return. However, that seems like a bit of overkill in my case, as I mostly day sail. I just motor out of the harbor, set sails, then return, and motor to the slip. However, as to my offshore Channel Island trips? That's a different story... Maybe there is something I haven't thought about, so I'd like to hear your reasoning on this. Thanks

[deckofficer]

A couple of reasons for me. My dedicated battery compartment only has the space for a single group 24 battery or in my case (4) Winston 100 ah cells in series. I put the "return" pack in the forward hatch. The main reason is I always want enough juice to return to my launch point and if I get carried away on cruising I can get 40 miles out on a single pack, that would mean a lot of paddling if I didn't have a fresh pack to swap in for the return.

[Wireless1]

Originally Posted by deckofficer

A couple of reasons for me. My dedicated battery compartment only has the space for a single group 24 battery or in my case (4) Winston 100 ah cells in series. I put the "return" pack in the forward hatch. The main reason is I always want enough juice to return to my launch point and if I get carried away on cruising I can get 40 miles out on a single pack, that would mean a lot of paddling if I didn't have a fresh pack to swap in for the return.

Ah...I get it now. The difference here being a sailboat. Thanks.

[Maine Sail]

Wireless,

The Mastech will work fine BUT run it about 20% below its rating and it will be much happier. This means if you want a 20A charger buy a 30A or 40A unit...

Be sure you set the voltage into a full pack as there will be voltage drop in the cable when under high load, which can mean an over shoot when full... I find a wall timer works well to plug the power supply into to shut it down, when you figure out how long it takes to charge.....

[Wireless1]

Originally Posted by Maine Sail

Wireless,
The Mastech will work fine BUT run it about 20% below its rating and it will be much happier. This means if you want a 20A charger buy a 30A or 40A unit...
Be sure you set the voltage into a full pack as there will be voltage drop in the cable when under high load, which can mean an over shoot when full... I find a wall timer works well to plug the power supply into to shut it down, when you figure out how long it takes to charge.....

Thanks Maine Sail,
In my searching it looks like Mastech only makes a 20 Amp unit at 60 volts that comes with the (EX) over-voltage/current protection features. So if I go with them, I'm stuck with their HY6020EX 20 amp model. Darn... That means when charging at 16 amps output I will be charging my 80% DOD 320AH pack at .05C --- a crawl at about 16 hours to get back to full. My mind pictures a small squirrel cage going round n' round... Maybe I'll look into a big "dumb" charger and just do some kind of voltage cut off to turn off the charger using a solid state AC relay, or your timer idea will work fine too.

Also, I'm not understanding what you mean when you say;
>> be sure you set the voltage into a full pack as there will be voltage drop in the cable when under high load, which can mean an over shoot when full...<<

The first full charge will be with the pack bottom balanced to 2.6 volts (41.6 volts). If my target is 3.6 volts per cell, or 57.60 volts for the pack, then what would you suggest I set the output voltage and over-protection on the Mastech? If I set it for 57.6 volts going to the pack, and there is a voltage drop in the cables going to the pack, how could that possibly create an over charge?
BTW, I will be monitoring voltage at the pack terminals with my DVM. Thanks for your help, and sorry I'm confused on this aspect.

[Maine Sail]

Originally Posted by Wireless1

Also, I'm not understanding what you mean when you say;
>> be sure you set the voltage into a full pack as there will be voltage drop in the cable when under high load, which can mean an over shoot when full...<<

The first full charge will be with the pack bottom balanced to 2.6 volts (41.6 volts). If my target is 3.6 volts per cell, or 57.60 volts for the pack, then what would you suggest I set the output voltage and over-protection on the Mastech? If I set it for 57.6 volts going to the pack, and there is a voltage drop in the cables going to the pack, how could that possibly create an over charge?
BTW, I will be monitoring voltage at the pack terminals with my DVM. Thanks for your help, and sorry I'm confused on this aspect.

If you start at a 20A load there will be a voltage difference between the power supply and the battery terminal. As the battery charges, and current falls to 5A at XX voltage you will now have a slight overshoot if you had set the voltage at full load.... Adjust your voltage into a light load then when the bank is full it will be accurate. These power supplies don't have dedicated voltage sensing circuits and only measure voltage over the charging leads.......

[Wireless1]

Originally Posted by Maine Sail

If you start at a 20A load there will be a voltage difference between the power supply and the battery terminal. As the battery charges, and current falls to 5A at XX voltage you will now have a slight overshoot if you had set the voltage at full load.... Adjust your voltage into a light load then when the bank is full it will be accurate. These power supplies don't have dedicated voltage sensing circuits and only measure voltage over the charging leads.......

Ah...I see. Thanks very much for this. I'm beginning to think I just need to get a larger capacity charger and work out some means of controlling it. Or perhaps at least get one that I can disable the bulk, absorption, float stages.
Thanks once again for your help.

[deckofficer]

Originally Posted by Wireless1

Ah...I see. Thanks very much for this. I'm beginning to think I just need to get a larger capacity charger and work out some means of controlling it. Or perhaps at least get one that I can disable the bulk, absorption, float stages.
Thanks once again for your help.

This is the charger I use. Small enough not to overload a 15 amp 115 VAC circuit and weighs about 10 lbs.
KP-C(1200W) Charger [KP-C(1200W)] - $209.00 : EV Assemble, LiFePO4, Electric Bike Conversion Kit, EV Charger, BMS, EV Components, EV Parts, All for EV!

[westwinds]

If you still have a running diesel engine, I would plot the engine RPM against speed through water. (no wind or current, speed run in both directions and take an average speed) Then find the propeller curve for the boat engine and look at the horsepower vs. RPM and finally plot the horsepower vs. speed. You could of course do the same thing with a table. What you will find is that the curve is not linear. If you double the horsepower you do not come close at all to doubling the speed. So, at lower speeds, you get much better range. At a power draw of 10.5 KWH, your range will be approximately 10 miles. Someone may want to check my numbers. I assumed hull speed and 80% efficiency. With the above table or graph, you might determine that you want a smaller electric motor for a much better range. Another option is to put a speed controller on the motor for your present design and develop the above graph for different power settings and speed. You could also make a graph of range and speed using the amp-hours available with the batteries you have selected. My last thought is you may want a generator to extend your range, probably of smaller horsepower than the electric motor where the batteries supply extra power for short periods with the extra thrust is needed.

The above calculations assume no wind and current. A calculation using GPS speed and power setting for actual conditions at the actual time of the sail would be a better idea, but for design purposes, the above graphs would be a good idea.

[Wireless1]

Originally Posted by deckofficer

This is the charger I use. Small enough not to overload a 15 amp 115 VAC circuit and weighs about 10 lbs.
KP-C(1200W) Charger [KP-C(1200W)] - $209.00 : EV Assemble, LiFePO4, Electric Bike Conversion Kit, EV Charger, BMS, EV Components, EV Parts, All for EV!

Bob,
What is your overall opinion of the quality of these units? The KP-K 2000 watt charger looks like an amazing deal, especially since it can be set to a custom setting depending on what your particular LiFePO4 cells need.
Thanks very much for pointing me in this direction. I've sent them an email with a few questions...before I pull the trigger on one.