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  #1  
Old 04-18-2017, 10:49 AM
Kingpen Kingpen is offline
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Default Help picking a BMS

hello all- I'm kind of new to this and I feel like i'm missing something. I'm building a pack of 72 cells in series and I wanted to get something to balance the cells and let me monitor the individual voltages. The only thing I've come across are the Orion and Lithiumate - both of which are $2000+/- solutions that seem to be the Cadillac's of BMS systems. Is there anything more affordable? I really only need balancing and something to see the voltages.
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  #2  
Old 04-18-2017, 08:35 PM
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madderscience madderscience is offline
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Default Re: Help picking a BMS

AFAIK your options are those you mentioned and the Manzanita Micro BMS (which is also going to end up at a similar price). With 72 cells the incremental price per cell is $30 which certainly adds to the overall cost, but it is worth it to protect your pack. Some people go without BMS but they instead have to obsess over manual balancing and monitoring of the pack to avoid damage. It is possible to DIY a BMS (I've done it) but it is a lot of work.

a dirt simple thing you could do for monitoring which is better than nothing is put a 12V digital panel voltage display (powered by the voltage it samples) on each bank of 4 cells. You can buy these for a few dollars each on ebay. Unless you both a very high and a very low cell in one bank you will be able to see the voltage deviation and act on it.

For simple balancing, you can make a post-zombie-apocalypse LiFePO4 balancer circuit from 5 1n4001 diodes and a 1a minature fuse in series (total forward voltage drop about 3.5v) which will drag down higher voltage cells faster than ones at a lower voltage. This style of balancer is leaky (~10ma at 3.4v IIRC) vs the microcontroller powered ones all the major BMSs use which probably draw <1ma when "off", but if you are going to be using and charging the pack regularly it should be a nonissue for large format cells. (10ma would take 400 days to drain a 100ah cell, but in reality by the time the cell hits 3.0v the diode based balancer would be almost completely "off" so they could not totally drain the cells)

You would still want to check the voltmeters near the top of their charge often and use a fairly slow charger with a fairly conservative HVC.

I have this exact described setup which has been running perfectly 3-4 years now powering my tool shed (24v, 60ah pack) and charged off a solar array.

Good luck.
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Last edited by madderscience; 04-18-2017 at 08:45 PM.
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Old 04-19-2017, 02:04 AM
Duncan Duncan is offline
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Default Re: Help picking a BMS

You need to think about what a BMS does

If you start with a balanced pack then I see almost nobody who has had any drift - however I have had cells fail

So I believe that you need
(1) Balanced pack
(2) Some way of detecting a failed cell

I'm using a "Lee Hart - Batt Bridge
http://www.evdl.org/pages/battbridge.html
This will show if one side of your pack is behaving differently than the other - or you could split the pack into smaller units

Otherwise be careful about going to the extremes of full charge and full discharge
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Old 04-19-2017, 07:37 AM
Moltenmetal Moltenmetal is offline
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Default Re: Help picking a BMS

http://elithion.com/lithiumate_pro_controllers.php

Not an endorsement- I've never used it, but it seems more economical than other options now that the miniBMS is no longer sold.

If you're using Li ion batteries, you either need a BMS or YOU will be the BMS. And you'll get lazy sometime. Buying a BMS is cheap insurance.
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Old 04-20-2017, 02:21 PM
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Default Re: Help picking a BMS

Zeva also sell nice low cost BMS, but don't show the per cell voltage.
http://www.zeva.com.au/
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Old Yesterday, 07:48 AM
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Default Re: Help picking a BMS

Quote:
Originally Posted by madderscience View Post
AFAIK your options are those you mentioned and the Manzanita Micro BMS (which is also going to end up at a similar price). With 72 cells the incremental price per cell is $30 which certainly adds to the overall cost, but it is worth it to protect your pack. Some people go without BMS but they instead have to obsess over manual balancing and monitoring of the pack to avoid damage. It is possible to DIY a BMS (I've done it) but it is a lot of work.

a dirt simple thing you could do for monitoring which is better than nothing is put a 12V digital panel voltage display (powered by the voltage it samples) on each bank of 4 cells. You can buy these for a few dollars each on ebay. Unless you both a very high and a very low cell in one bank you will be able to see the voltage deviation and act on it.

For simple balancing, you can make a post-zombie-apocalypse LiFePO4 balancer circuit from 5 1n4001 diodes and a 1a minature fuse in series (total forward voltage drop about 3.5v) which will drag down higher voltage cells faster than ones at a lower voltage. This style of balancer is leaky (~10ma at 3.4v IIRC) vs the microcontroller powered ones all the major BMSs use which probably draw <1ma when "off", but if you are going to be using and charging the pack regularly it should be a nonissue for large format cells. (10ma would take 400 days to drain a 100ah cell, but in reality by the time the cell hits 3.0v the diode based balancer would be almost completely "off" so they could not totally drain the cells)

You would still want to check the voltmeters near the top of their charge often and use a fairly slow charger with a fairly conservative HVC.

I have this exact described setup which has been running perfectly 3-4 years now powering my tool shed (24v, 60ah pack) and charged off a solar array.

Good luck.
Thanks, I actually did plan on doing something similar to this. I'm just going to use some simple volt meters to watch/compare voltages across several cells in series. It would have been nice to be able to see individual cells at a glance but that seems to be to expensive for what I'm doing. Charging was an issue at first but I think i sorted that out and it involves using a parallel board with one of my high powered hobby chargers. Should be able to charge and balance all of them at once.
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Old Yesterday, 07:49 AM
Kingpen Kingpen is offline
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Default Re: Help picking a BMS

Quote:
Originally Posted by Yabert View Post
Zeva also sell nice low cost BMS, but don't show the per cell voltage.
http://www.zeva.com.au/
Those actually do look to be one of the cheapest but is still very expensive once I buy all of the 12s boards. Looks like they 3d print the cases too, kind of cool.
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Old Yesterday, 07:49 AM
Kingpen Kingpen is offline
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Default Re: Help picking a BMS

Quote:
Originally Posted by Moltenmetal View Post
http://elithion.com/lithiumate_pro_controllers.php

Not an endorsement- I've never used it, but it seems more economical than other options now that the miniBMS is no longer sold.

If you're using Li ion batteries, you either need a BMS or YOU will be the BMS. And you'll get lazy sometime. Buying a BMS is cheap insurance.
These guys actually came out to be the most expensive of all.
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Old Yesterday, 06:01 PM
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Default Re: Help picking a BMS

Thunderstruck will be offering their new BMS soon
http://www.thunderstruck-ev.com/bms/

Call them and ask for "early adopter" discount

Their system scales nicely to 72 cells, in multiples of 12 per LTC chip.

EVen better if you get their charger too, it integrates with BMS natively over CANBUS
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