Accurate LiFEPO4 state of charge monitoring

[Dr.Ben]

New member and question here, please. I'm am doing a lithium swap on a '98 Club car to replace the 48V Pb acid boat anchors. From other interests, I am well versed in Li batteries to include the point of this post. This a DIY pack. Unlike LiION and even Lipo batteries, you cannot tell SOC of a LiFEPO4 battery with voltage; the discharge curve is flat til dead. Thus you need an amp hour counter. I'm sure I'm not the only one here who has done a DIY Li swap AND knew the detail about SOC monitoring, so I'd like to here what you have used for a AH counter.

Thanks much!
Ben Minor

[cybercart]

to replace the 48V Pb acid boat anchors.

Thanks for the chuckle

I think the most common issues encountered are related to BMS, whether the regen simply over juices the batteries leading to bms shutdown or there is too much pull from any mods doing the same. What kind of BMS will you be using and if not do you plan to self balance? You can check what kind of setup you have using this chart, since usually an OBC & charger interlock bypass are needed, and here is a link to the most recommended SOC meter

Welcome to the group!

[Dr.Ben]

Thanks for the chuckle

I think the most common issues encountered are related to BMS, whether the regen simply over juices the batteries leading to bms shutdown or there is too much pull from any mods doing the same. What kind of BMS will you be using and if not do you plan to self balance? You can check what kind of setup you have using this chart, since usually an OBC & charger interlock bypass are needed, and here is a link to the most recommended SOC meter

Welcome to the group!

Thank you for the warm welcome and valuable advice. It is most appreciated. Now I have direction to follow on monitoring instrumentation. To answer your questions, I'm going to build the pack as an 8S1P+8S1P configuration. My other hobby of large radio controlled electric helicopters (7000 watt power systems on 50 volt Lipos). I have on hand a two 2 port lithium charger which will handle this chemistry in particular and balance the two eight cells. I can charge each side at about 15amps and get it done in a sensible time frame. I can set the cut off a little below peak to cover for any little bit of regen that might occur early on in the run event. After I see what the peak current is pulling a mild grade, I can appropriately fuse the system for safety.

Kind regards,
Ben

[Michael Eddie]

Welcome Doc,

Your configuration and charging plans sound well thought out, and your RC helicopter hobby must be exciting. Do you happen to have an OBC bypass already in place or plans to implement one?

Edit- Oops, I see cybercart already beat me to it

[Dr.Ben]

Man you folks are kind to newcomers. Much appreciated! So here's my question.....if everything associated with charging of the lithium retrofit is external, will the OBC still stick its proverbial nose in and disrupt things ? It's easy enough to bypass, but do I need to since I'll be using custom charge and balance pigtails?

Thanks much,
Ben

[Michael Eddie]

So basically around 1997, Club Car introduced the concept of relocating a component typically housed within the charger into the golf cart itself, inventing the OBC. However issues arise during charger transitions, as the OBC loses its ability to detect activity leading to confusion which shutdowns the controller. This is increasingly prevalent now as more cart owners upgrade their beloved older club cars to lithium/upgraded batteries. Turns out Club Car made the decision to eliminate the OBC entirely around 2014 rendering issues during charger swaps a thing of the past.

[Dr.Ben]

Understood, but if all the system will ever see in my case is a charged battery managed completely external to the original charge circuity and will never see LVC voltage, will the OBC even notice anything and revert to shutdown?

Ben

[Michael Eddie]

The OBC operates by gathering data using the original charge plug and charge system during charge cycles. However, when the original charge plug is diverted, the OBC is unable to receive this information. As a result, the OBC cannot perform its intended functions. This can cause the OBC to disable the controller rendering the golf cart inoperable. If battery management is handled externally, the OBC won't receive the necessary data from the charge cycles. This lack of data can indeed cause the OBC to assume something is wrong and potentially shut down the controller as a safety measure.

In your case, rerouting the electrical connections so that the OBC is no longer part of the circuit will disable the OBC from interfering with your new charging setup and eliminates the possibility of it disabling the controller due to a lack of information. At this point, you can safely remove the OBC entirely.

The topic of an OBC bypass has been quite the hot topic here for years. There are debates on whether the OBC is still necessary after the bypass is completed. The answer is that it depends. Those sticking with the original charger need to keep the OBC in place because it manages the charge cycles and keeps everything running smoothly. For those keeping their lead acid setup but upgrading to a different charger, you can keep the OBC in the system since it also controls the battery light function. However, if you're making the leap to lithium/LiFEPO4 batteries, you can bid the OBC farewell. Lithium battery setups work best with lithium programmed shunt driven state of charge meters, which takes over the OBC's role completely.

My apologies if my response isn't directly addressing your questions, Doc. I'm grateful you brought up this topic, and I wanted to highlight this information as it's often overlooked in DIY upgrade guides. Feel free to ask any questions you might have or enlighten us with your wisdom!

[Dr.Ben]

The OBC operates by gathering data using the original charge plug and charge system during charge cycles. However, when the original charge plug is diverted, the OBC is unable to receive this information. As a result, the OBC cannot perform its intended functions. This can cause the OBC to disable the controller rendering the golf cart inoperable. If battery management is handled externally, the OBC won't receive the necessary data from the charge cycles. This lack of data can indeed cause the OBC to assume something is wrong and potentially shut down the controller as a safety measure.

In your case, rerouting the electrical connections so that the OBC is no longer part of the circuit will disable the OBC from interfering with your new charging setup and eliminates the possibility of it disabling the controller due to a lack of information. At this point, you can safely remove the OBC entirely.

The topic of an OBC bypass has been quite the hot topic here for years. There are debates on whether the OBC is still necessary after the bypass is completed. The answer is that it depends. Those sticking with the original charger need to keep the OBC in place because it manages the charge cycles and keeps everything running smoothly. For those keeping their lead acid setup but upgrading to a different charger, you can keep the OBC in the system since it also controls the battery light function. However, if you're making the leap to lithium/LiFEPO4 batteries, you can bid the OBC farewell. Lithium battery setups work best with lithium programmed shunt driven state of charge meters, which takes over the OBC's role completely.

My apologies if my response isn't directly addressing your questions, Doc. I'm grateful you brought up this topic, and I wanted to highlight this information as it's often overlooked in DIY upgrade guides. Feel free to ask any questions you might have or enlighten us with your wisdom!

That was a flawless reply and perfectly answered my questions. I'll bypass the OBC and move forward. I'll post pics when I'm done the conversion too.

Thanks so much for the education!

Ben

[cybercart]

Edit- Oops, I see cybercart already beat me to it

Looks like I'm one step ahead this time