How to add a BMS to lipo batteries?

[Arbol]

From the thread:

http://www.pedelecs.co.uk/forum/threads/battery-advice-please.17548/#post-216573

I could see several pedelecers use lipo hardpacks such as this 4s 5Ah:

http://www.hobbyking.co.uk/hobbyking/store/__26801__Turnigy_5000mAh_4S1P_14_8v_20C_hardcase_pack_UK_Warehouse_.html?strSearch=4s hard

The more I read about lipos, it seems that apart from punctures (risk aleviated by the hard pack) and bad connections (coming from connect and disconnect at every charge), a big risk is that a battery in parallel gets damaged for any reason, damaging the rest of the batteries in the pack. Instead, if there is no parallel connection (only series) the risk diminishes a lot (eg a main fuse should be able to control that risk).

So, let me assume I have enough with 5Ah, so no parallel connection. Then, I select 12s, so I need a 3x connection in series.

Let me assume I build a 3x series harness, such as the following (just an example, this is for 2x series, 3x series harness is not available at HK, I believe):

http://www.hobbyking.com/hobbyking/store/__18604__HXT4mm_Battery_Harness_14AWG_for_2_Packs_in_Parallel.html?strSearch=parallel

using HXT 6mm:

http://www.hobbyking.com/hobbyking/store/__20353__HXT_6mm_Gold_Connector_w_Protector_10pcs_set_.html?strSearch=hxt

(a technician at HK has told me these bullets are the best connections, better than XT90 http://hobbyking.com/hobbyking/store/__25662__XT90_Battery_Harness_10AWG_for_2_Packs_in_Parallel.html, for example)

as well as:

http://www.hobbyking.com/hobbyking/store/__9677__Turnigy_Pure_Silicone_Wire_8AWG_1mtr_RED.html?strSearch=8awg

or

http://www.hobbyking.com/hobbyking/store/__9671__Turnigy_Pure_Silicone_Wire_10AWG_1mtr_RED.html?strSearch=10awg

So, if I build a 3x series harness, from the output point of view, I have created a "big 12s battery".

However, the balance wire presents a problem:

A 12s BMS has a balance wire plug with 12+1 pins (12s plus ground). In principle, this is plug and play with a big, expensive 12s lipo battery. However, if I consider a "synthetic" 12s battery built out of 3x 4s lipo batteries, each JST-XH of a 4s pack has 5 pins (4 + 1 ground). So in total 15 pins.

My question is then: is it possible to "connect" the three 4s JST-XH of the three 4s lipo batteries into a single male 12s JST-XH, which connects into the female 12s JST-XH of the BMS? can I share the three grounds, of the three batteries, into a single one, the single remaining pin of the BMS?

If this could be done, it would be relatively simple, no need to dismantle cables to charge, no spot welding, and cheap.

Mistakes / suggestions for improvement?

 

[Deleted member 4366]

Yes it can be done. 12S chargers are a bit more expensive though. There was a thread on ES a couple of months ago about 12S chargers and how to wire the 12S balance connector.

 

[Arbol]

I asked BMSbattery if they could provide a 12s bulk charger, and they said yes, just take a 13s "standard" bulk charge and write a note in the order to be set up at 12s.

 

[Arbol]

Just a tentative answer to my question about how to plug three 4s JST-XH connectors into a 12s JST-XH connector at the BMS:

use a parallel board (ironically, since the connection here is in series, not in parallel), as in:

http://endless-sphere.com/forums/viewtopic.php?f=14&t=52240

where it says:

"Let's hook up the balance cables."

Also, that thread explains how to do a harness for a parallel connection.

 

[Arbol]

I assume the link d8veh refers to is:

http://endless-sphere.com/forums/viewtopic.php?f=14&t=57221#p853835

right?

 

[drsolly]

From the thread:

http://www.pedelecs.co.uk/forum/threads/battery-advice-please.17548/#post-216573

I could see several pedelecers use lipo hardpacks such as this 4s 5Ah:

http://www.hobbyking.co.uk/hobbyking/store/__26801__Turnigy_5000mAh_4S1P_14_8v_20C_hardcase_pack_UK_Warehouse_.html?strSearch=4s hard

The more I read about lipos, it seems that apart from punctures (risk aleviated by the hard pack)

I use those, and I think they're perfect. I also use this:
http://www.ebay.co.uk/itm/Field-Box-Shoulder-Strap-Storage-Hunting-Ammo-Fishing-Kit-Lunch-Box-Army-Surplus-/231191162157?pt=UK_SportingGoods_Hunting_ShootingSports_ET&hash=item35d4112d2d

It's a hard plastic, and it's *exactly* the right size to take eight of the 4s 20c hardcase batteries; I put a bit of foam at the bottom to soften the jolt when I go over bumps. And if I fall, the batteries are protected a *lot* more than the very thin hardcase covering.

and bad connections (coming from connect and disconnect at every charge), a big risk is that a battery in parallel gets damaged for any reason, damaging the rest of the batteries in the pack. Instead, if there is no parallel connection (only series) the risk diminishes a lot (eg a main fuse should be able to control that risk).

So, let me assume I have enough with 5Ah, so no parallel connection. Then, I select 12s, so I need a 3x connection in series.

I put a pair of those batteries in series; I actually tape them together. And I have seven pairs. When one is exhausted, I plug in another pair, and so on.

I'm not worried about bad connections. I use EC5 connectors; those are connected to the 4mm bullets that come with the battery. If the connectors ever get damaged, I can just solder up another EC5-to-4mm connector.

Most of the connector damage comes from the big inrush current when you connect the battery to the controller. That can very easily be avoided - see my other post about putting a resistor across the switch.

Let me assume I build a 3x series harness, such as the following (just an example, this is for 2x series, 3x series harness is not available at HK, I believe):

http://www.hobbyking.com/hobbyking/store/__18604__HXT4mm_Battery_Harness_14AWG_for_2_Packs_in_Parallel.html?strSearch=parallel

using HXT 6mm:

http://www.hobbyking.com/hobbyking/store/__20353__HXT_6mm_Gold_Connector_w_Protector_10pcs_set_.html?strSearch=hxt

(a technician at HK has told me these bullets are the best connections, better than XT90 http://hobbyking.com/hobbyking/store/__25662__XT90_Battery_Harness_10AWG_for_2_Packs_in_Parallel.html, for example)

I looked at those. I prefer EC5 connectors. I think, because they were easier to plug/unplug.

as well as:

http://www.hobbyking.com/hobbyking/store/__9677__Turnigy_Pure_Silicone_Wire_8AWG_1mtr_RED.html?strSearch=8awg

or

http://www.hobbyking.com/hobbyking/store/__9671__Turnigy_Pure_Silicone_Wire_10AWG_1mtr_RED.html?strSearch=10awg

EC5s will take 8AWG. I use 12AWG.

So, if I build a 3x series harness, from the output point of view, I have created a "big 12s battery".

However, the balance wire presents a problem:

A 12s BMS has a balance wire plug with 12+1 pins (12s plus ground). In principle, this is plug and play with a big, expensive 12s lipo battery. However, if I consider a "synthetic" 12s battery built out of 3x 4s lipo batteries, each JST-XH of a 4s pack has 5 pins (4 + 1 ground). So in total 15 pins.

My question is then: is it possible to "connect" the three 4s JST-XH of the three 4s lipo batteries into a single male 12s JST-XH, which connects into the female 12s JST-XH of the BMS? can I share the three grounds, of the three batteries, into a single one, the single remaining pin of the BMS?

NO!!!!

What you call "grounds" are actually the negative end of the three batteries. When you connect the three batteries in series, those three "grounds" are at 0v, 14.4v and 28.8v, and if you connect them together, you'll get a *massive* short circuit which will burn out your balance wires (and may do a lot more dmage besides).

There is a way to connect three 4s JST-XHs to a single 12s JST-XH, but the way you suggested isn't it.

If this could be done, it would be relatively simple, no need to dismantle cables to charge, no spot welding, and cheap.

Mistakes / suggestions for improvement?

My way is simple, and you don't need a BMS. Your balance charger performs that role when charging, and the battery monitors http://hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=42953 perform that role when discharging.

 

[Arbol]

Thanks very much for your insightful comments.

When you say "you don't need a BMS": a BMS is handy to charge, with a bulk charger is enough, and no cables have to be disconnected / connected.

I now understand your method, and probably is best for example to test. But I would like the "comfort" of having a BMS for normal usage.

When you say "There is a way to connect three 4s JST-XHs to a single 12s JST-XH, but the way you suggested isn't it": may I ask which method is it?

 

[Arbol]

Tentative answer to my last question above:

something like this:

http://www.hobbyking.com/hobbyking/store/__51406__XH_Multi_Pack_Balance_Board_2_10S.html

but 12s (HK does not have it, I believe).

 

[drsolly]

There's a thread on ES about it, but I can't remember where, use the search function. Basically, a 4s has five wires; 0, 3.7, 7.4, 11.1 and 14.4 v. When you connect two 4ses in series, you have to work out which of those five wires on one cable is at the same voltage as which of those five wires on the other, and connect those two. Similarly for three 4ses.

Yes, the HK board would let you convert two 4s. But that doesn't solve the problem of three 4s.

 

[Deleted member 4366]

You start at the bottom of the first battery using 1a to 5a. The 1b on the second battery is the same as the 5a on the first, so you join them together. 2b to 5b are the next 4 pins. Pin 1c is the same as 5b, so you join them. That leaves 2c to 5c.

The sequence is therefore
1a 2a 3a 4a (5a +1b) 2b 3b 4b (5b +1c) 2c 3c 4c 5c, which makes 13 pins.

 

[Arbol]

OK, let me see if I have understood correctly:

For the first 4s battery:

Pin 1=cell 1 negative, at 0V
Pin 2=cell 1 positive / cell 2 negative, at 3.7V
Pin 3=cell 2 positive / cell 3 negative, at 7.4V
Pin 4=cell 3 positive / cell 4 negative, at 11.1V
Pin 5=cell 4 positive, at 14.8V

For the second battery:

Pin 6=cell 1 negative, at 14.8V
Pin 7=cell 1 positive / cell 2 negative, at 18.5V
Pin 8=cell 2 positive / cell 3 negative, at 22.2V
Pin 9=cell 3 positive / cell 4 negative, at 25.9V
Pin 10=cell 4 positive, at 29.6V

and the third battery would be the same, up to 44.2V.

The only critical issue is pin 5 and pin 6 have to be joined, such that they occupy the same pin at the BMS.

Here one could wonder about setting a 14.8V to pin 6: could that be too much voltage? But from physics class we know voltage in itself is irrelevant, only change in voltages matter. So we can set pin 6 at 14.8V without problems.

Is that correct?

 

[Deleted member 4366]

Correct.

 

[Arbol]

OK, now the next step: batteries in parallel. We have been discussing batteries in series are quite safe, since a single fuse suffices. However, with batteries in parallel, things become more complicated:

I have a comment a ES which refers to this quite nicely, I copy & paste:

"(...) using the information from the following link:

http://www.allaboutcircuits.com/vol_1/chpt_11/5.html

First I consider the following statement from the link:

"For our series battery bank, one fuse will suffice to protect the wiring from excessive current, since any break in a series circuit stops current through all parts of the circuit"

This statement seems to suggest that having a main fuse is a good idea, since with only a fuse at the discharge cable level is enough to deal with shorts in a series construction. This is probably why many builds have only a main fuse: in this very simple way, builders are able to handle a big array of potential problems.

However, not all problems can be solved by a main fuse:

"With a parallel battery bank, one fuse is adequate for protecting the wiring against load overcurrent (between the parallel-connected batteries and the load), but we have other concerns to protect against as well. Batteries have been known to internally short-circuit, due to electrode separator failure, causing a problem not unlike that where batteries of unequal voltage are connected in parallel: the good batteries will overpower the failed (lower voltage) battery, causing relatively large currents within the batteries' connecting wires. To guard against this eventuality, we should protect each and every battery against overcurrent with individual battery fuses, in addition to the load fuse"

So, this statement is suggesting we should have fuses at the cell individual level."

At the ES post, I was thinking about 18650 cells. Having fuses at a 18650 cell level is quite complicated. There is a thread about this same issue, but the final answer is a DIY still cannot do that easily.

However, with lipos maybe things are different. For a 3s2p configuration for 4s batteries (44V, 10Ah), having a fuse for each 4s battery does not seem so difficult. Something like this:

Could normal fuses be added to each of the six 4s batteries in a 3s2p configuration, such that if one of them fails for any reason, the fuse just disconnects that battery from the whole thing?

Sure, the biker would start notice the bike does not work that well, and LVC would be hit much earlier than usual. But that would be a good signal a battery has to be changed.

But apparently, the risk of a parallel connection for lipos would be overcome.

However, it is not clear to me what would happen to the balance wire of a given cell if the fuse of that cell is activated.

Ideas?

 

[drsolly]

This doesn't affect things using my method. I use two 4s in series; when that's exhausted, I swap to another pair of 4s in series. Unplug two EC5s, plug in two EC5s (and the associated battery monitors on the balance leads) and I'm ready.

 

[Arbol]

Yes, the more I think about it, the more I believe it is better to avoid parallel connections and stick with series connections.

With a BMS, it should be possible to use your method, modified a bit, right? The plugging and unplugging of two EC5s remains unchanged with respect to your method. What would change is that one should take another pair of balance wires (with female and male connectors at both ends, something like http://www.hobbyking.com/hobbyking/store/__9737__JST_XH_4S_Wire_Extension_20cm_10pcs_bag_.html), plug the male connector to the BMS (and leave it there always; in fact, it would not be the male connector, but the bare wires, since we have had to use the trick that pin 5 of the first battery is connected to the pin 6 of the second battery, so it is hard to plug / unplug that), and plug the balance wires of the battery into the female of the new balance wires. This last connection is the one that should be plugged / unplugged to replace batteries.

Does it make sense?

Edit: in addition to the above, there is a connection between the negative of the first battery and the BMS. There should be also an intermediate connector to be able to disconnect the battery, while at the same time leaving the connection with the BMS in a permanent state.

 

[drsolly]

If you're planning to make 3 4ses into a 12s, and run it with a bms, then you'll have to find some way to connect those balance leads into the bms. Maybe you can find a bms that accepts three 4s balance leads, or maybe you have to solder together a 12s balance lead.

I'm glad that I don't have this problem!

 

[Deleted member 4366]

Once you connect two lipos in parallel, they become one of twice the capacity. Connect the balance leads as well as the power leads. It's not at all complicated. One fuse in series with the load is adequate. There's no need for any more.

You're making everything too complicated.

 

[Arbol]

Once you connect two lipos in parallel, they become one of twice the capacity. Connect the balance leads as well as the power leads. It's not at all complicated. One fuse in series with the load is adequate. There's no need for any more.

You're making everything too complicated.

Yes, I realize I make things too complicated. I do not do it in purpose. It is the way I think.

I feared about the comment above:

"Batteries have been known to internally short-circuit, due to electrode separator failure, causing a problem not unlike that where batteries of unequal voltage are connected in parallel: the good batteries will overpower the failed (lower voltage) battery, causing relatively large currents within the batteries' connecting wires. To guard against this eventuality, we should protect each and every battery against overcurrent with individual battery fuses, in addition to the load fuse"

But as you say, two lipos in parallel become one of twice the capacity. In fact, a 4s lipo is (I guess) 4 elementary (3.7V) cells in parallel, so connecting two 4s lipos is basically akin to having a 8s cell.

As a consequence, yes, a lipo in parallel may internally short-circuit, but in fact what will happen is an elementary cell short-circuits, and this happens independently if the lipos are in parallel or not.

So, yes, I can parallel the batteries.

Just crossing the fingers that if something wrong happens, the fuse is activated.

 

[Arbol]

If you're planning to make 3 4ses into a 12s, and run it with a bms, then you'll have to find some way to connect those balance leads into the bms. Maybe you can find a bms that accepts three 4s balance leads, or maybe you have to solder together a 12s balance lead.

I'm glad that I don't have this problem!

Yes, this is part of the complexity of adding a BMS. I agree your method is simpler at the building step. My concern is about charging. But yes, I am not saying the BMS method is better or worse than yours, simply different for different purposes.

BTW, probably I would go with 2x 6s rather than 3x 4s. Reasons: it is easier to build a 2s series harness than a 3s one, and dnmun at ES said hardpacks were not good.

 

[shemozzle999]

From the thread:

http://www.pedelecs.co.uk/forum/threads/battery-advice-please.17548/#post-216573

I could see several pedelecers use lipo hardpacks such as this 4s 5Ah:

http://www.hobbyking.co.uk/hobbyking/store/__26801__Turnigy_5000mAh_4S1P_14_8v_20C_hardcase_pack_UK_Warehouse_.html?strSearch=4s hard

The more I read about lipos, it seems that apart from punctures (risk aleviated by the hard pack) and bad connections (coming from connect and disconnect at every charge), a big risk is that a battery in parallel gets damaged for any reason, damaging the rest of the batteries in the pack. Instead, if there is no parallel connection (only series) the risk diminishes a lot (eg a main fuse should be able to control that risk).

So, let me assume I have enough with 5Ah, so no parallel connection. Then, I select 12s, so I need a 3x connection in series.

Let me assume I build a 3x series harness, such as the following (just an example, this is for 2x series, 3x series harness is not available at HK, I believe):

http://www.hobbyking.com/hobbyking/store/__18604__HXT4mm_Battery_Harness_14AWG_for_2_Packs_in_Parallel.html?strSearch=parallel

using HXT 6mm:

http://www.hobbyking.com/hobbyking/store/__20353__HXT_6mm_Gold_Connector_w_Protector_10pcs_set_.html?strSearch=hxt

(a technician at HK has told me these bullets are the best connections, better than XT90 http://hobbyking.com/hobbyking/store/__25662__XT90_Battery_Harness_10AWG_for_2_Packs_in_Parallel.html, for example)

as well as:

http://www.hobbyking.com/hobbyking/store/__9677__Turnigy_Pure_Silicone_Wire_8AWG_1mtr_RED.html?strSearch=8awg

or

http://www.hobbyking.com/hobbyking/store/__9671__Turnigy_Pure_Silicone_Wire_10AWG_1mtr_RED.html?strSearch=10awg

So, if I build a 3x series harness, from the output point of view, I have created a "big 12s battery".

However, the balance wire presents a problem:

A 12s BMS has a balance wire plug with 12+1 pins (12s plus ground). In principle, this is plug and play with a big, expensive 12s lipo battery. However, if I consider a "synthetic" 12s battery built out of 3x 4s lipo batteries, each JST-XH of a 4s pack has 5 pins (4 + 1 ground). So in total 15 pins.

My question is then: is it possible to "connect" the three 4s JST-XH of the three 4s lipo batteries into a single male 12s JST-XH, which connects into the female 12s JST-XH of the BMS? can I share the three grounds, of the three batteries, into a single one, the single remaining pin of the BMS?

If this could be done, it would be relatively simple, no need to dismantle cables to charge, no spot welding, and cheap.

Mistakes / suggestions for improvement?

Lipo's are designed to deliver very high current output for model engines which can exhaust the entire battery in less than 10 minutes.

Dedicated 12s batteries and 12s balanced chargers are already available, mainly for helicopter applications, and are very expensive. They do not need to have BMS's but rely on the electronic speed controller driving the motor to safely control the battery power output.

I can not see any advantage in using them on ebikes unless you are seeking to save weight.

Many ebikers are worried about the cost of replacement batteries but put into perspective, it probably costs about £1 a trip over the life of a battery given the number of charges now available from a battery.

You just need to factor in this when buying a bike and set up a savings account and feed in a monthly deposit based on your monthly usage so that funds are available when are replacement is required, this will effectively give you a new bike as the battery technology would have improved over the time you have used the old one.