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Good Chinese BMS review
#11
(03-07-2019, 12:40 AM)Sean Wrote:
(03-06-2019, 11:41 PM)ajw22 Wrote:
(03-06-2019, 04:30 PM)Sean Wrote: What criteria does it use to deem the battery to be fully charged ?

Charging is blocked when any cell reaches "Overvoltage" (default 4.2V?, me: 4.15V) OR the overall voltage reaches a set level (default 59.5V? .....

You are aware that just reaching a certain voltage isnt the complete picture in terms of charge completion criteria ?


Yes, and that is the job for the separate CC/CV charger, which in my case will eventually be configured to charge to 4.05V * 14.  I skipped mentioning that part because I have not installed my final charger yet.  
So unless the packs are significantly unbalanced (either in capacity and/or SoC), the BMS cell overvoltage protection should not trigger at all.
After all, the primary function of this (any?) BMS is to ensure safety by keeping voltages, currents and temperatures within the safe range, and secondarily to maximize usable battery capacity by keeping the packs in balance.

This BMS app does show a SoC in Ah and % using some algorithm involving Ah in/out (and voltage? It's configurable with the PC app), but I have not fully explored that yet.
BTW, the smartphone app is apparently designed for use on electric bicycles/vehicles and shows the current speed as well as expected remaining range.

"Nominal Capacity: 160Ah" <- that's my guesstimate input, not determined by the BMS.





(03-07-2019, 01:17 AM)Ebarnett Wrote: Thanks.  Looks like I may go either batrium.  I want the flex ability to upgrade and not pay for s bums sgsin


Not wanting to paying for another BMS when upgrading is not really an argument considering the Batrium system costs more than 10x to begin with.
With Batrium, you get somewhat easier expandability (just splice in the new packs / longmons), while this BMS requires you to reattach the 2 power cables and 8 balancing leads to the new BMS (still only a 30min job).

I think the biggest arguments for Batrium is all the logging, statistics with pretty graphs, remote control and monitoring, and ability to trigger external relays.
Modular PowerShelf using 3D printed packs.  30kWh and growing.
https://secondlifestorage.com/t-AJW22-s-...PowerShelf
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#12
(03-07-2019, 01:52 AM)ajw22 Wrote: I think the biggest arguments for Batrium is all the logging, statistics with pretty graphs, remote control and monitoring, and ability to trigger external relays.

..... and the ability to fully charge correctly.

A BMS should be able to signal to any attached charger a number of states during charge, including requesting a limited rate request when in bypass to prevent thermally overloading the bypass resistors, and a cease charge current (completely) when cell voltage and tail current settings have been achieved. Ending charge based on cell voltage, and/or pack voltage only will result in a chronically under charged battery and an inability to make full use of the available capacity.

Obviously a lower cost BMS can't be expected to include every feature included in a BMS costing a few factors more, but readers might regard the ability to fully charge their battery an important deciding factor to consider when selecting  a BMS and charger.

I dont recall tail current ever being mentioned on SLS, I suspect very few are fully aware of how important a factor it is in a charging system.
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#13
(03-07-2019, 07:36 AM)Sean Wrote: ..... and the ability to fully charge correctly.

A BMS should be able to signal to any attached charger a number of states during charge, including requesting a limited rate request when in bypass to prevent thermally overloading the bypass resistors, and a cease charge current (completely) when cell voltage and tail current settings have been achieved. Ending charge based on cell voltage, and/or pack voltage only will result in a chronically under charged battery and an inability to make full use of the available capacity.

Obviously a lower cost BMS can't be expected to include every feature included in a BMS costing a few factors more, but readers might regard the ability to fully charge their battery an important deciding factor to consider when selecting? a BMS and charger.

I dont recall tail current ever being mentioned on SLS, I suspect very few are fully aware of how important a factor it is in a charging system.


For powerwalls with lithium ion (the apparent focus of this forum) and EV use, you almost always WANT a chronically under charged battery to maximize cycle life.
eg. Tesla strongly recommends to charge only to 90% (Elon said 80%) for regular use. That implies ca 4.0~4.1V/cell. Only when the extra range is needed should one charge to 100% (at the sacrifice of cycle life).
The Powerwall2 specs says "14kWh, 13.5kWh usable", most likely because it does not fully charge it in order to extend its cycle life.
And Toyota/Honda hybrid traction batteries are said to charge/discharge only in the 40%~80% range.
You really only want to charge to the maximum on small portable devices with perfectly matched cells, and even so I've had several laptops (Lenovo?) with the option to stop charging at ca 90%.
Particularly with second life batteries with a mix of manufacturers, chemistry and of unknown cycles left, you want to be gentle to the cells by not charging them to 100%.

"Tail current": that's just a different name for the CV charge stage, where current gradually drops to near 0 (the graph looks like a tail). Since powerwalls are usually paired with solar/wind power, and the charger knows best whether the battery is not taking the current or the charger simply cannot supply more current, I'd argue that the charger is in charge (lol) of displaying the "fully charged" indicator and to stop charging (or switch to float mode).

Thermally overloading bypass resistors: this Chinese BMS has just 50mAh bypass current, hardly enough to overheat anything. A problem with a complex and costly solution engineered away by being a cheap ass. But it does block the charging when the balancing system needs time to catch up (ie. cell overvoltage trigger), and that gets flagged and counted in the app. Now, the puny 50mA balancing this BMS provides is certainly inferior to the Batrium Longmons with their 2 Amps of bypass current. But so far 50mA has proven to be plenty for my use.

Some cutting edge EV BMS (paired with CHAdeMO, Tesla Supercharger) can request a lower charge current when the whole pack starts to overheat (#RapidGate), but I haven't heard of any such feature for overheating bypass resistors. I'd like to know your source on this one.
thanar and kwabdul like this post
Modular PowerShelf using 3D printed packs.  30kWh and growing.
https://secondlifestorage.com/t-AJW22-s-...PowerShelf
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#14
There's a few misconceptions in your previous post, I'll address just two.

Under charging, and reducing the terminal voltage are two separate points: you are correct to say that best practise (to maximise number of cycles) is to reduce the terminal voltage (let's say to 4.05/cell) - but whatever terminal voltage you opt for, that cell will still be undercharged unless you are able to monitor that cells tail current and amend/cease at the recommended level (which is usually a few percentage points of the cells C rating) - a CC/CV charger can only do that based on overall pack voltage and current.

Thermal damage will quickly occur to bypass components unless additional dissipation is applied and/or a charge rate reduction is applied when cells are in bypass. Without thermal management even 50mA will damage components if they see an excessive temperature. 

I'm unsure what your EVSe and in vehicle charging comment relates to. Rapidgate refers to overall pack temperature, not bypass component ratings.

Bypass component temperature monitoring is implemented in more advanced management systems (including the Batrium WM4) which enables external cooling to be applied as required, which enables the overall battery charge current to be maintained at a higher rate (when in bypass). In the absence of additional cooling, more advanced management systems are able to tell the charge system (perhaps via CAN) to reduce its rate of charge, such that the bypass components are able to function without the risk of thermal damage.

As an example, my home system is able to bulk charge at rates in excess of 150A, but when in bypass (without active cooling) the maximum rate needs to be reduced to below 10A.
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#15
Perhaps you can link or name that BMS that does cell level tail current monitoring. I'd like to read up on that.
For second life storage powerwalls, I doubt anyone cares whether all the cells were precisely charged to the targeted 4.05V, or undercharged to just 4.03V, or even overcharged to 4.07V. After all, this is the crowd that bunches 2000mAh and 2099mAh cells together praying that randomness will somehow balance that out.

Bypass thermal damage: I only have a low res picture of the BMS, but it looks like a discrete FET and several resistors. I can only assume that the components are properly sized (with extra head room) to handle 50mA with just passive cooling even under "higher" environmental temperatures. At least that's what any sane engineer with hopes of continued employment would.

RapidGate: Exactly, I've never heard of a bypass component overheat event causing the charger to lower the current. Throw an error code and/or stop charging (if temporary) maybe. Again, please cite that BMS+charger you're referring to.

Active cooling on demand on Batrium WM4: That would be done via the external relay control I praised in post #11?

"more advanced management systems are able to tell the charge system (perhaps via CAN) to reduce its rate of charge". Again, what BMS+charger? Is it available for consumer purchase? Are you not referring to automotive components you can't even get the datasheet for unless you are a senior engineer at a major automobile company and sign a NDA?
Modular PowerShelf using 3D printed packs.  30kWh and growing.
https://secondlifestorage.com/t-AJW22-s-...PowerShelf
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#16
All of the features I've mentioned are included in typical higher cost management systems within the DIY sector, one being the current Batrium product, WM4.

Charge rate control is implemented for a number of differing manufacturers products, as detailed here ... https://www.batrium.com/pages/faq-remote

My home system uses WM4/Victron Venus on GX/Multiplus ....

Thread drift is evident, my original point was related to how a BMS should correctly signal charge complete - I'm not expecting a cost effective system to have the same features as a wallet buster.

(03-07-2019, 03:11 PM)ajw22 Wrote: For second life storage powerwalls, I doubt anyone cares whether all the cells were precisely charged to the targeted 4.05V, or undercharged to just 4.03V, or even overcharged to 4.07V.  After all, this is the crowd that bunches 2000mAh and 2099mAh cells together praying that randomness will somehow balance that out.

You keep referring to under or over charging based on a voltage ...... that's not the full picture, hence my references to /cell tail current.

(03-07-2019, 03:11 PM)ajw22 Wrote: RapidGate: Exactly, I've never heard of a bypass component overheat event causing the charger to lower the current.  Throw an error code and/or stop charging (if temporary) maybe.  Again, please cite that BMS+charger you're referring to.

Lowering charge current based on system component temperature is an expected, very desirable feature. As per the Leaf, these are self protective systems designed into the system

Batrium mons are individually temperature monitored, via the control logic the numerous available outputs can be configured to act on the numerous signals available  ..... cooling required, heating required, charge complete, in bypass, supply under/over voltage - there's a long list, and theres a mixture of outputs.

I'm not trying to convince anyone that your favourite BMS isnt a wise investment, I'm simply trying to point out some of the subtle, but vital, additional features available on alternative systems.
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#17
Thank you for clearing that up.
Clearly the BMS I'm reporting on is designed to be just a cheap, standalone safety device, not part of a whole suite of interconnected devices to optimally use the battery.

Right, I should have said:
For second life storage powerwalls, I doubt anyone cares whether all the cells were precisely charged to the targeted 90%, or undercharged to just 88%, or even overcharged to 92%.
Modular PowerShelf using 3D printed packs.  30kWh and growing.
https://secondlifestorage.com/t-AJW22-s-...PowerShelf
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#18
I hate to ask a stupid question but I'm going to.  Ajw22, you mention charging to only 90% of cells max voltage.  On a battery with a Standard charge of .5C of 1500ma and 4.2v wouldn't the max charge voltage need to be .9 x 4.2 = 3.78v per cell.  How did you come up with 4.05v.  That comes to about 98% not 90%.  Please show me where my math is incorrect.

Thanks.
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#19
I just picked 4.05v as an arbitrary voltage below 4.1v/100% as a conversation point - voltage isnt an accurate method of determining Li chemistry cells SoC btw.
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#20
Thanks Sean, I've see some other posts recommending 4.0 as the max charge voltage to help with battery life.
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