Hello everyone,
I am a senior engineering student whose graduation project is a Mars rover-style robot which is to be powered via an 8S8P Li Ion battery pack (33.6V 24Ah) that was assembled from these 18650 batteries. The battery pack is managed by this BMS (10Series 60Amps).
Currently I am facing 2 problems:

1. I can't get the BMS to work, because it needs to be charged first, which leads me to problem number 2;
2. I don't have a way to charge the battery.

The problem lies with the fact that I live in a country where no fancy Li-Ion chargers exist, my only option of getting them would be to order them online, wait for the duration of the shipping and customs procedures (weeks to months), and risk the fact the product could be refused entry (which has happened with a group of colleagues), all while paying an exorbitant amount of money. This is to say that unfortunately I have arrived at a not-so-engineeringly-sound list of alternative solutions that I was hoping to get some information on.
again I want to preface this by saying that I know these are not the optimal solutions, It's just that I have no other options as far as I'm aware:

• Solution A: Use one of these DC Adapters, and step up the voltage through a boost converter, depend on the BMS to handle the charging process of the cells. I am not sure how well this would work, and also the current rating is very low (highest I could find was 3A at 24V. or 4.7A at 19v)
• Solution B: Same as solution A but use one of these DC adapters, and step down the voltage through a step down converter, again the same problem with the current (only much worse this time.)
• Solution C: Use a switching power supply such as this one, and decrease the output voltage or use a step down converter to decrease it. However; as far as we've researched this is absolutely not recommended due to the switching nature of regular SM power supplies.
• Solution D: Use a laptop's charger such as this one, cut off the end (cannot find a 7.4x5mm female plug where I live for the life of me) and use it with a step up converter (main benefit here would be the higher current than the DC adapters above.) However; I have seen mixed opinions online about this approach.
• Solution E: Something else entirely that we have not thought of.

Sorry for the long post, I am looking for suggestions/ recommendations. Any help would be greatly appreciated.

Thank you in advance!

Note: This robot is for the European rover challenge competition. Where the focuses is more on the mechanical, communications, and autonomy challenges of being in a Mars-like/ desert environment rather than recreating a Mars Rover one-to-one. Thus, using a solar panel is not really viable or recommended.

 

1. I can't get the BMS to work, because it needs to be charged first, which leads me to problem number 2;
2. I don't have a way to charge the battery.

Are you saying that, once you get enough initial charge on your batteries, that the BMS would kick in?

If that's the problem, there are a couple tricks you can use. One is to get the BMS hooked up and then briefly - just a tap - place a DC source in parallel to the (dead) battery pack. This will fool the BMS into turning on the charge current and continuing. The other trick is to charge your battery pack slowly by "manual" means, up to a level where again the BMS will recognize it and kick in. By manual charging I mean a DC source with adequate voltage applied to the battery pack through a resistor (or light bulb) to limit the current to well below 1C. You have to keep an eye on it and let the BMS take over as soon as it can. You don't want to risk overcharging your batteries.

 

Are you saying that, once you get enough initial charge on your batteries, that the BMS would kick in?

If that's the problem, there are a couple tricks you can use. One is to get the BMS hooked up and then briefly - just a tap - place a DC source in parallel to the (dead) battery pack. This will fool the BMS into turning on the charge current and continuing. The other trick is to charge your battery pack slowly by "manual" means, up to a level where again the BMS will recognize it and kick in. By manual charging I mean a DC source with adequate voltage applied to the battery pack through a resistor (or light bulb) to limit the current to well below 1C. You have to keep an eye on it and let the BMS take over as soon as it can. You don't want to risk overcharging your batteries.

Thank you for the reply!
To be completely honest I am not 100% sure at what point the BMS would kick in, however through previous experience with BMSs (although of a different kind) and looking up online, charging the BMS was the most recommended thing to get it to start working.
I understand the two suggestions that you proposed, I am kind of leaning towards the first one just to test if the BMS can get to work from it early on, other wise I guess option 2 is the only way to go.
Do you have any recommendations/ preferences for how to charge the battery pack once the BMS is working?

 

Do you have a decent 10A+ bench power supply with current limit at your university/college.
Set the power supply current limit to a reasonable value and voltage to 33.6V.
Connect the batteries via the BMS.
The cells will charge in a controlled manner assuming the BMS does it’s thing and the
charge current will drop to a very low value indicating the end of the charge cycle.

 

Do you have any recommendations/ preferences for how to charge the battery pack .once the BMS is working?

Depends on the BMS!

LiPo batteries need to be charged at a constant current from a source that exceeds the required final voltage by a small amount until the cells reach the defined terminal voltage, then switches to constant terminal voltage until the current drops to, typically, 0.01C. Whether the BMS does the current limiting or expects the external supply to do it is one of the big questions. When the BMS senses that an individual cell has reached, or is near, terminal voltage, it reduces the charge rate on that cell (by bypassing with a low value resistor - passive balancing - or transferring power up or down the stack - active balancing), allowing other cells to continue charging - ideally it reduces the overall charge current but obviously if the charge rate is set externally that becomes an issue. Once balancing has started the charging switches to constant voltage at the overall terminal voltage for the pack and the current reduces as the cells collectively reach their final voltage.

So the big question is what does the BMS need? Just a voltage source a volt or two above terminal volts with sufficent current to meet your charging requirement? Or does it need external current limiting/constant voltage provision. I have used BMS of both types but what does yours do? Post a link...

 

Do you have a decent 10A+ bench power supply with current limit at your university/college.
Set the power supply current limit to a reasonable value and voltage to 33.6V.
Connect the batteries via the BMS.
The cells will charge in a controlled manner assuming the BMS does it’s thing and the
charge current will drop to a very low value indicating the end of the charge cycle.

Thank you for your reply.
That is a great suggestion, I had not looked into that. I did some research and it seems like a commonly used method.
Unfortunately I am not sure of the availability of such power supplies in my faculty. I have found one at a local supplier but it is slightly below the required voltage (it can output a maximum of 32V and 6A). Do you have any idea if using a boost converter to get to 33.6v would impose any risks?
I realize the 6Amps will mean a pretty slow charging time, but this is the best option I have right now.

 

Depends on the BMS!

LiPo batteries need to be charged at a constant current from a source that exceeds the required final voltage by a small amount until the cells reach the defined terminal voltage, then switches to constant terminal voltage until the current drops to, typically, 0.01C. Whether the BMS does the current limiting or expects the external supply to do it is one of the big questions. When the BMS senses that an individual cell has reached, or is near, terminal voltage, it reduces the charge rate on that cell (by bypassing with a low value resistor - passive balancing - or transferring power up or down the stack - active balancing), allowing other cells to continue charging - ideally it reduces the overall charge current but obviously if the charge rate is set externally that becomes an issue. Once balancing has started the charging switches to constant voltage at the overall terminal voltage for the pack and the current reduces as the cells collectively reach their final voltage.

So the big question is what does the BMS need? Just a voltage source a volt or two above terminal volts with sufficent current to meet your charging requirement? Or does it need external current limiting/constant voltage provision. I have used BMS of both types but what does yours do? Post a link...

Thank you for your detailed reply and thorough explanation.
I am not quite sure how to determine what will be the exact behavior of my BMS; this is the link of the BMS that I have (you can find the closest thing to a datasheet that they provide by scrolling down the page) but I have not been able to find anything useful regarding charging.
I think based on the replies that I have received both here and on other forums that the best course of action will be to use a bench power supply at a low current and hopefully that along with the BMS can help ensure that things go smoothly.

 

Purchase two of these power supplies and connect them in series.
Set power supply 1 to say, 20V and current limit to 5A and use power supply 2, also set to 5A to accurately set the overall voltage to 33.6V. Connect the outputs to your pack via the BMS.
It will take a while to charge a fully discharged pack but on the other hand at that current you will have time to monitor, with a dvm, the terminal voltage of the pack and of individual cells to check progress.
Also the power supplies will be useful additions to your tool set.

 

Purchase two of these power supplies and connect them in series.
Set power supply 1 to say, 20V and current limit to 5A and use power supply 2, also set to 5A to accurately set the overall voltage to 33.6V. Connect the outputs to your pack via the BMS.
It will take a while to charge a fully discharged pack but on the other hand at that current you will have time to monitor, with a dvm, the terminal voltage of the pack and of individual cells to check progress.
Also the power supplies will be useful additions to your tool set.

That's a great suggestion, thank you so much!

 

That's a great suggestion, thank you so much!

Yes, it is, but an alternative could be...

https://ram-e-shop.com/product/kit-smps-48v-7-2a-fan/
https://ram-e-shop.com/product/dc-d...tage-converter-with-color-lcd-voltmeter-copy/

which slightly cheaper and will give you approx 44v @ 7A

Regarding your BMS, it appears to do the constant current + balancing itself, though the balancing current is a paltry. So you only need to supply a fixed voltage supply at a suitable wattage. The supporting app looks good from the little info given.

I have a couple of concerns:
1.
It seems that on the 60A discharge BMS, the charge current appears to be fixed at 40A and its not obvious that can be changed in the app. According to the battery manufacturer's spec, those cells should only be charged at 2A or 16A for a 8P configuration. They don't have the thermal mass to be charged much harder. I certainly wouldn't go above 1C/3A, or 24A for the pack. Obviously with only 7A to play with from that PSU there's no risk of thermal runaway on 8P. However I don't know how that BMS will play if it can't reach its programmed constant current. Ideally it will accept it, but it might construe that as a fault and shut down.

2.
Because the external charger is connected directly to the battery+ its not clear how, or whether, the BMS controls overall pack voltage for the CV part of the charging regime. There is no spec for the supply voltage to the BMS.

 

Yes, it is, but an alternative could be...

https://ram-e-shop.com/product/kit-smps-48v-7-2a-fan/
https://ram-e-shop.com/product/dc-d...tage-converter-with-color-lcd-voltmeter-copy/

which slightly cheaper and will give you approx 44v @ 7A

Regarding your BMS, it appears to do the constant current + balancing itself, though the balancing current is a paltry. So you only need to supply a fixed voltage supply at a suitable wattage. The supporting app looks good from the little info given.

I have a couple of concerns:
1.
It seems that on the 60A discharge BMS, the charge current appears to be fixed at 40A and its not obvious that can be changed in the app. According to the battery manufacturer's spec, those cells should only be charged at 2A or 16A for a 8P configuration. They don't have the thermal mass to be charged much harder. I certainly wouldn't go above 1C/3A, or 24A for the pack. Obviously with only 7A to play with from that PSU there's no risk of thermal runaway on 8P. However I don't know how that BMS will play if it can't reach its programmed constant current. Ideally it will accept it, but it might construe that as a fault and shut down.

2.
Because the external charger is connected directly to the battery+ its not clear how, or whether, the BMS controls overall pack voltage for the CV part of the charging regime. There is no spec for the supply voltage to the BMS.

Hello, sorry for the late reply; finals have been hectic.
Just want to say thank you for taking the time to point out a possible alternative; that was very kind of you. Funny enough I had not seen your reply but this was the exact option that we went with at the end.
We just bought it today and we are yet to try charging the battery, I will provide updates here with what we reach.
Regarding your concerns:
1. That is something that I did not think we'd have to worry about. Guess the only way to find out is through testing. Problem is that the CC/CV stepdown that we found can only output a max of 5A not 15A as was listed (the 15A one was out of stock)
2. I guess we will depend only on the CC/CV stepdown converter for that then, with constant monitoring for any irregularities.

 

Update, got the charging circuit to work using these components:
https://ram-e-shop.com/product/kit-smps-48v-7-2a-fan/ attached to https://ram-e-shop.com/product/dc-d...k-voltage-converter-with-color-lcd-voltmeter/

The cells are being charged and balancing, however the BMS is not giving the correct output. More details can be found here:
https://forum.allaboutcircuits.com/threads/bms-not-working-properly.194433/