Jump to content

Buffers


zlymex

Recommended Posts

I don't know how to call these but they act as soft cushion to reduce current.
webwxgetmsgimg.thumb.jpg.77347db20db9a645a15980fed85548ac.jpg
 

1. Spark preventer

When connecting the battery to the main board, there is always a spark because there is a large capacitor on the main board that often at 0 volt, since the voltage on a capacitor cannot be changed suddenly, the current spike is very large limited only by the internal resistance of the capacitor, the internal resistance of the battery, and wire resistance.

Inserting an 1k resistor will limit the current below 0.1A thus eliminate the spark. The tao value will be 2.4 seconds(for Msuper 3) therefore the charge time will be 7.2 seconds(3*tao). I added a LED allowing the charge to be monitored as well. once the light goes off, the buffer can be removed and reconnect directly.
595bba9ee58bf_webwxgetmsgimg(2).thumb.jpg.ef05c5f6d7ebcc0693347879eced9da7.jpg
595bba9d267f1_webwxgetmsgimg(1).thumb.jpg.a311729cb8c63f93f3b2a80953e2ad81.jpg
595bbaa19c1ed_webwxgetmsgimg(3).thumb.jpg.a7cf57e13c087b712f12ae16c4e12a34.jpg

 
2. Balancing adaptor

When the battery pack is unbalanced to a certain degree, the charge current will be cut off by the BMS completely because some cell has reached to 4.25V(thus trigger the charge protection) even if some other cells are at low voltage. This will make the balancing circuit never functioning. 

 If, however, the charge current is equal to or less than the balancing current, no cell will be reach to 4.25V because the current will be consumed by the balancing resistor(no extra current to charge the cell with 4.2V or little higher).

I made a 19mA current limiter for IPS EUCs just for this purpose. If a cell is under 4.2V, it will be charged by this 18mA current. If a cell has reached 4.2V, the balancing circuit of that cell will start bypassing this current and make the voltage stay at 4.19V to 4.20V.
19mA.gif.f889632abda55c023d7a684f74213074.gif
595bbb64410ea_webwxgetmsgimg(4).thumb.jpg.274379431a3294a2dbe8d3d3820488cd.jpg
595bbb65c83a5_webwxgetmsgimg(5).thumb.jpg.036ecdadf8c5f33f6ffe237e5dd7eb43.jpg

Note: the balancing resistors in an IPS EUC is 220 Ohm making the balancing current of 19mA(=4.2V/220 Ohm). The balancing charge may take days since this current is small.


3. Current limiter for external battery pack

There are situations when somebody run out of battery, and there is a fully charged external battery pack available. However, because there is a large voltage difference, the pack cannot be inserted onto the EUC directly to avoid over current. This problem can be solved by inserting a current limiter in between. I use two 6V 20W halogen bulbs for the job and can be switch on one or two.
bulbs.gif.c3df9c57f3edd2cc5cf64d12475661fc.gif
595bbc1f0c6fa_webwxgetmsgimg(6).thumb.jpg.51d9fc1dba01b6a12149ef6df55352d6.jpg
595bbc23c724d_webwxgetmsgimg(7).thumb.jpg.63dd6535e170c4aec07f030a4e3f016d.jpg
595bbc25e2f64_webwxgetmsgimg(8).thumb.jpg.2d5a7ee1ad2df3656c00e01ae8ea7319.jpg

Link to comment
Share on other sites

11 minutes ago, zlymex said:

When the battery pack is unbalanced to a certain degree, the charge current will be cut off by the BMS completely because some cell has reached to 4.25V(thus trigger the charge protection) even if some other cells are at low voltage. This will make the balancing circuit never functioning. 

Is this common to all wheels or specific to IPS? I was under the impression that the balancing scheme in the typical BMS's shunts (bypasses) the cells that have reached full voltage (around 4.20V), but continue charging the rest, so the fully charged cells won't raise their voltage further, and the rest will catch up.

Link to comment
Share on other sites

14 minutes ago, esaj said:

Is this common to all wheels or specific to IPS? I was under the impression that the balancing scheme in the typical BMS's shunts (bypasses) the cells that have reached full voltage (around 4.20V), but continue charging the rest, so the fully charged cells won't raise their voltage further, and the rest will catch up.

IPS EUCs are very prone to unbalance because the cell quality is not very good.

If unbalance to a certain degree(severe), for instance, 4 cells is 3.9V, 4.0V, 4.1V, 4.2V, added up to 16.2V, less than 16.8V nominal, therefore the charger will produce the full current say at 2A, this will make the last cell reach to 4.25V soon, and the first cell is still at 3.95v. This will trigger the charge protection of the BMS and cut off the charge current completely, and there is no more balancing current, and the first cell(and 2nd, 3rd)  have no chance to catch up.

Edit: the bypassed current is the balacing current, which is small(19mA in the case of an IPS EUC), much smaller than the normal charge current.

Link to comment
Share on other sites

23 minutes ago, zlymex said:

IPS EUCs are very prone to unbalance because the cell quality is not very good.

If unbalance to a certain degree(severe), for instance, 4 cells is 3.9V, 4.0V, 4.1V, 4.2V, added up to 16.2V, less than 16.8V nominal, therefore the charger will produce the full current say at 2A, this will make the last cell reach to 4.25V soon, and the first cell is still at 3.95v. This will trigger the charge protection of the BMS and cut off the charge current completely, and there is no more balancing current, and the first cell(and 2nd, 3rd)  have no chance to catch up.

Edit: the bypassed current is the balacing current, which is small(19mA in the case of an IPS EUC), much smaller than the normal charge current.

Yes, I understood the problem there, but is this common to all wheels, or just for IPS? I thought something like this is common to the passive-balancing BMS's (they've apparently used LiFePo4's as an example, because they say the maximum voltage is 3.65V), bypassing and/or discharging the higher charged cells while charging:

Passive cell balancers

Also called bleeding cell balancers or dissipative cell balancers. Resistors are used to bleed the energy from the good cells, in order to match the voltage to those of the bad cells. It is clear that this is wasting a lot of energy because the good cells are in the majority.

 

Passive cell balancing circuit Passive cell balancing circuit

 

 

Bleeding cell balancing BMS with 390Ω bleeding resistors Bleeding cell balancing BMS with 390Ω bleeding resistors

 

The balancing procedure can proceed as follow:

  1. Initially, the balancer is turned off after the battery charger is connected.
  2. When any battery cell reaches 3.65V, the BMS will turn on the balancing circuit in this channel, and the discharge resistor in this channel will slightly drain the cell.
  3. When any cell reaches 3.9V, the BMS will turn off the charge current by the power transistor in the charge line.
  4. At that time, all the cells whose voltages are higher than 3.65V will be still being drained by the discharge resistors in their channels. This way, all the high cells will be discharged to 3.65V.
  5. When the highest cell drops to around 3.7V, the BMS will connect the charger again.
  6. Note that, at this time, there are cells that are still below 3.65V.
  7. Return to 2.

Finally, all cells are fully charged to 3.65V.

 

The passive balancing hardware is implemented with a bypass resistor and a switch across every cell in the pack. The balancing resistor is typically used in one of two ways (Figure 2, below). It can be used to steer charging current around the cell so that batteries with a lower state of charge (SOC) can charge at a higher rate and remain charging without risk of overcharging and damaging cells with a high SOC. Optionally the resistor can be used to bleed excess charge from batteries with higher charge states to equalize them with batteries having a lower SOC.
 

linearbalance2.gif
Figure 2: Two options for passive cell balancing. Resistor value determines the primary function.
 

The primary hardware design concern is to determine the appropriate balancing current, which is set by the value of the bypass resistor. The required balancing current largely depends on the capacity of the cell, the amount of time that can be allowed for balancing, the expected amount of imbalance, and how the resistor will be used. If used to bypass the charger current, several amps will be shunted. If the balancing resistor is used to bleed excess charge, the resistors will be sized to meet the desired balancing time.

The passive balancing is only capable of correcting SOC imbalance stemming from pack loading due to the battery monitoring circuitry, and cell self discharge and internal resistance effects. If constantly monitored, these sources should only create small amounts of imbalance on a day-to-day basis. The BMS system for this lab evaluation has a balancing resistor of 33 ohms that sets the balancing current to roughly 100mA, a large balancing current for small batteries, but one that allows balancing operations to take a shorter amount of time.

 

Link to comment
Share on other sites

12 hours ago, esaj said:

Yes, I understood the problem there, but is this common to all wheels, or just for IPS?

My knowledge about batteries of other EUCs(apart from IPS and Gotway) is quite limited therefore I cannot give an definite answer to this question. What I can say is that I experience many cases of severe unbalancing of IPS EUCs including 3 times on my own T260(which is my first EUC). However, severe unbalancing never happened to my three Gotway EUCs probably they use quality cells, or I've paid more attention to the charging, or they use smaller balancing resistors(120 Ohms).

 

12 hours ago, esaj said:

The balancing procedure can proceed as follow:

  1. Initially, the balancer is turned off after the battery charger is connected.
  2. When any battery cell reaches 3.65V, the BMS will turn on the balancing circuit in this channel, and the discharge resistor in this channel will slightly drain the cell.
  3. When any cell reaches 3.9V, the BMS will turn off the charge current by the power transistor in the charge line.
  4. At that time, all the cells whose voltages are higher than 3.65V will be still being drained by the discharge resistors in their channels. This way, all the high cells will be discharged to 3.65V.
  5. When the highest cell drops to around 3.7V, the BMS will connect the charger again.
  6. Note that, at this time, there are cells that are still below 3.65V.
  7. Return to 2.

This turn-on and turn-off repeat cycle may or may not happen, depending on the type of BMS used. That may well be the case for Gotway EUCs where they use descret componets(but I didn't test it, the problem-free of my gotway batteries seems to support this). However for IPS EUCs, once the cut out happened, it may not turn on again because they use smart ICs(MAX14921) for BMS, and the unbalanced situation cannot be recovered by prolonged charge.
P1020481ms.thumb.jpg.76118a554c8aadbd3984590aa16c1d93.jpg

Chart below is one of the case(cell voltage distribution) for a T260 where cell 16 is over charged and triggered the charge protection while cell 14 is way out.
yang1.gif.55e6765269969860befe37c547aebb9d.gif

Even for simple BMS such as link below, the overcharge release voltage is 4.15V, lower than the balancing release voltage(4.18V), that means the overcharge may not be released as a result of bleeding current, thus the turn-on will reluctant to occurs, at least we have to wait for a very long time so that the voltage drop from 4.18V to 4.15V because of very small leakage current.
https://www.aliexpress.com/item/16S-60V-or-67-2-V-unicycle-lithium-ion-battery-BMS-60V-li-ion-battery-protection/32422681053.html

Furthermore, the voltage difference for Li-ion battery from the balancing start(4.18V or 4.20V) to over-charge protection voltage(4.25V) is small(0.05V to 0.07V), the time for low voltage cells to catch up is limited for each turn-off and turn-on cycle, if the cycle does exist. While in the case of LiFePo4 batteries, the difference is many times larger(3.9V-3.65V=0.25V).

Link to comment
Share on other sites

Archived

This topic is now archived and is closed to further replies.

×
×
  • Create New...