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CatNewBee · 01-09-2018, 08:40

Quote:

Originally Posted by Dockhead

You have more technical training and knowledge than I do, so I don't think there is any point in my trying to give a lecture based on my own reading. Better you should do the reading and give ME a lecture.

For whatever it's worth -- this system does not seem complex to me, if you just forget about the architecture of your lead power system and put a clean sheet of paper in your mind.

What "additional home made box"? It's just a couple of contactors. The concept is very simple, and designing and wiring up such a system is well within even my modest technical abilities.

As John61 said, HVC and LVC are last-ditch protection, not daily battery management. For daily battery management, the BMS needs to have the ability not only to sound alarms, but to control the mains charger and the alternator. This is something which may be difficult for some, easy for others. With Balmar, Victron and Outback equipment, gear of that level, should be pretty straightforward.

Actually lead battery banks could use a LVC just as much as lithium could, and they exist ("battery savers"). If I were designing a new lead acid power system today, I would surely include one of these. This is not a complicated system and would prevent probably 80% of battery damaging events even for lead.

It is not the way it works, usually every charge system, like MPPT, Alternator, shore charger is responsible for its task, they all need to be configured to the right settings. The BMS is a smart fuse, based on cell voltage limits, it also takes care of balancing. If you do not trust your charge sources you can add logic to disconnect them or turn them off early. One option is to use configurable battery monitors like the Victron BMV712 to switch on SOC.

Some BMS have more options / signals then over voltage protection / unde voltage protection and can communicate warnings to chargers or loads to turn them off before ovp / lvp kicks in. Some BMS are CAN bus capable and can be networked that way to the chargers, but this is not yet very common.

**a64pilot** · 01-09-2018, 08:52

Yes lead batteries could use an LVC, except that I’m not that stupid to need one, and secondarily, I’d rather sacrifice the bank if I had to leave it, than the Boat. I’d rather kill my bank and keep the bilge pump operating.

Lead I can leave the boat with no issue, Solar gives it s little kick at absorption in the morning and quickly drops to float.

What system allows that in an LFP bank?
That is going to require you to build some kind of I assume relay box based on voltage to connect and disconnect the charge source, then you need apparently another to disconnect the bank.
Are they smart enough to know when Solar is working so that they aren’t connecting at night and disconnecting during the day? Then as you have pointed out, how do you handle the alternator?

It appears that it’s going to take not one homemade box, but several to cover all the bases, and then what’s the failure mode of each box and potential outcome?

I’m not saying Lithium isn’t and or won’t be better, just it’s still a Science Experiment, and will be until some Dr. or Accountant can buy them at West Marine and have a mechanic who is doing good to not mix pos and neg install them.

I have seen only seen one set of instructions on how to install Lithium, surely there must be others?

I’m seeing issues just from a curious look, issue that of course can be solved, but are best solved in my opinion by a purpose built box.
For example I’m sure people can and do build their own alternator controller, likely more easily programmable that a Balmar, bu the majority of us by a Balmar and install IAW the directions.

I think there need to be a similar “box” for Lithium, one that is robust and potted and you install IAW the directions, and include things like alarms that go off prior to LVC or HVC, allowing you ample warning that its coming and you can do something about to before it happens.

john61ct · 01-09-2018, 11:07

BMS is an overburdened term.

It really is a whole cluster of functionality designed to act as a failsafe protection to prevent the bank getting damaged.

When you buy "a BMS" you need to decide what additional functionality you want in order to optimize and automate day to day operations.

But many users do these functions themselves.

Some DIY the "final - layer" BMS functionality as well as the "usage" layers.

Lets just look at LVD on the way down, assuming bank-level only at 4S.

A. The BMS last-ditch failsafe may be set at 11.75V

An intelligent unit may distinguish between high-amp discharge at 11.65V and a low-amp cutoff at 11.85. But let's assume a single setpoint.

Now I would only want the most essential loads operating at 12.15V or below.

B. Refrigeration would cut out at 12.35V

C. The entertainment systems at 12.65V

etc

Only A. above is really BMS functionality.

____
Now OVD for charging.

Most LFP BMSs will cut off **way** higher than I would ever allow my bank to see.

I would have a global OVD set to 13.95V, and consider that BMS functionality.

A high-amp charge source may be allowed to get bank voltage to 13.90V, but only with "just stop" zero Absorb time.

Solar and other low current sources stop at 13.80V.

The BMS functionality is only ever used if a charge regulator fails.

john61ct · 01-09-2018, 11:09

And note none of the above involves "alarms".

If desired they can be added in, to go off before or concurrent with the circuit opening.

**newhaul** · 03-09-2018, 11:31

Question is how much did each of you spend on your BMS for the size of bank.
For 100ah I can find them from as cheap as $20 bucks to well north of a hundred.

john61ct · 03-09-2018, 11:51

All your protective gear to get your BMS functionality, more relevant for many DIYers.

If you don't want per-cell gear, then buying "a BMS" as such may not make sense.

Contactors, sensors, relays / solenoids, LVD / OVD gear, none of that is LFP specific.

And at high ampacities, can easily cost more than the cells themselves.

**newhaul** · 03-09-2018, 12:03

Quote:

Originally Posted by john61ct

All your protective gear to get your BMS functionality, more relevant for many DIYers.

If you don't want per-cell gear, then buying "a BMS" as such may not make sense.

Contactors, sensors, relays / solenoids, LVD / OVD gear, none of that is LFP specific.

And at high ampacities, can easily cost more than the cells themselves.

so basically I can just use the setpoints on my solar charge controller ( no alternator and all loads are being run via the load output on solar controller)
I know this is very specific to my setup.

travellerw · 03-09-2018, 12:15

Quote:

Originally Posted by newhaul

Question is how much did each of you spend on your BMS for the size of bank.
For 100ah I can find them from as cheap as $20 bucks to well north of a hundred.

My entire BMS and supporting gear initially cost $328USD (400ah bank). However I have added a second Blue Sea latching relay on the negative side which brought the cost to close to $500USD (for redundancy).

Eventually I will do a blog post with pictures and details on all the hardware I'm using.

**newhaul** · 03-09-2018, 12:21

Quote:

Originally Posted by travellerw

My entire BMS and supporting gear initially cost $328USD (400ah bank). However I have added a second Blue Sea latching relay on the negative side which brought the cost to close to $500USD (for redundancy).

Eventually I will do a blog post with pictures and details on all the hardware I'm using.

this is most helpful and a thread documenting it would be great.
So the good stuff seems to work out to about $125 per 100ah

travellerw · 03-09-2018, 13:45

Quote:

Originally Posted by newhaul

this is most helpful and a thread documenting it would be great.
So the good stuff seems to work out to about $125 per 100ah

BMS is a big subject and there are tons of options, plus those options even have options. Frankly we could have a 10 page post about everyone's BMS and why they went that way. (hint, look on Facebook there are groups with just that)

For me I set these requirements.
1. Low voltage/High voltage monitoring at the cell level (with disconnect)
2. Low voltage/High voltage monitoring at the pack level (with disconnect)
3. Catastrophic temperature monitoring on each cell (with disconnect).
4. Ability to isolate the pack for storage, maintenance or parked at a dock.
5. Audable alarms for pack level HV and LV
6. Ability to power the boat instruments and systems independant of the house bank from 2 different sources

So based on that.. I personally don't call my system a BMS (nor would the other cruisers I know with LFP). I call it a BP (battery protection). It doesn't really "Manage" the batteries (or cells), its simply a last line of defense if things go horribly wrong. All the management in my system is done at the charging sources. Technically, my cell level boards have the ability to balance the cells, but I will never go to a high enough voltage for that to kick in.

Right or wrong, that is what I choose and it cost me about $500USD to implement. Time will tell if it was right, but its based on quite a few other installs I have seen and documented.

**newhaul** · 03-09-2018, 13:50

Quote:

Originally Posted by travellerw

BMS is a big subject and there are tons of options, plus those options even have options. Frankly we could have a 10 page post about everyone's BMS and why they went that way. (hint, look on Facebook there are groups with just that)

For me I set these requirements.
1. Low voltage/High voltage monitoring at the cell level (with disconnect)
2. Low voltage/High voltage monitoring at the pack level (with disconnect)
3. Catastrophic temperature monitoring on each cell (with disconnect).
4. Ability to isolate the pack for storage, maintenance or parked at a dock.
5. Audable alarms for pack level HV and LV
6. Ability to power the boat instruments and systems independant of the house bank from 2 different sources

So based on that.. I personally don't call my system a BMS (nor would the other cruisers I know with LFP). I call it a BP (battery protection). It doesn't really "Manage" the batteries (or cells), its simply a last line of defense if things go horribly wrong. All the management in my system is done at the charging sources. Technically, my cell level boards have the ability to balance the cells, but I will never go to a high enough voltage for that to kick in.

Right or wrong, that is what I choose and it cost me about $500USD to implement. Time will tell if it was right, but its based on quite a few other installs I have seen and documented.

all I want / need is an hvc in the event of solar controller failure . On customers vessels were built in bms but the more I look the more I'm looking to build my own bank ( a heck of a lot cheaper )
lost an 8Dd that way due to over charging a long time ago .

Oceansailor · 03-09-2018, 13:57

Quote:

Originally Posted by travellerw

My entire BMS and supporting gear initially cost $328USD (400ah bank). However I have added a second Blue Sea latching relay on the negative side which brought the cost to close to $500USD (for redundancy).

Eventually I will do a blog post with pictures and details on all the hardware I'm using.

I’m in the process to install a 400Ah bank. I would appreciate if you could let me know the source for the parts you got for 328 USD?

travellerw · 03-09-2018, 14:03

Quote:

Originally Posted by Oceansailor

I’m in the process to install a 400Ah bank. I would appreciate if you could let me know the source for the parts you got for 328 USD?

Start looking here.. but understand this is a very DIY system (with the ability to modify the code in the head unit yourself). If I had to order again, I would probably order 2 head units for redundancy (possible not required if you pick a differen Blue Sea Relay)
https://evparts.com.au/ev-power-bms.html

After that I choose Blue Sea Latching relays. I went with the 7700, but there are other better options if you are ok with a tiny power draw (they will fail disconnect in the event of a head unit failure). The relays and extra hardware (temperature sensors, ect) were all ordered from Amazon.

john61ct · 03-09-2018, 18:59

Quote:

Originally Posted by newhaul

so basically I can just use the setpoints on my solar charge controller ( no alternator and all loads are being run via the load output on solar controller)
I know this is very specific to my setup.

Depends which one. Custom voltage at 13.75 possible?

Turn off temp comp?

What is the minimum possible Absorb Hold Time?

What will you do to prevent Float?