A tale of 2 BMS - Internal Vs External

brendanisatsea

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Hello all,
I'm in the design phase for a new Marine LiFePO4 system and have encountered two BMS Styles:

1) BMS that are "internal" to the battery and are wired so that the BMS is the NEG terminal output. (Daly, Overkill Solar etc)
2) BMS that are "External" to the battery and are only wired to the cells, not in line with the NEG terminal. (X2 BMS, Orion, REC)

What is the nature of this difference and what are the pros/cons to each style? It appears as though the external BMS can handle higher amperage, have CAN features to speak with Victron GX interfaces and are generally more expensive. Some seem to be ABYC approved/contain extra safety features.

Is there anything more to this difference?
 

Ampster

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An external BMS uses a contactor and an external current measuring device. My Orion has a heritage from EVs. It can give a fairly good measure of SOC by counting Coulombs. It can provide additional signaling to put a motor controller in turtle mode when SOC is low. It is more expensive than the internal BMSs. I have also built some 12 volt packs and used an Overkill BMS, but for a 42 kWh pack, I want the comfort and remote monitoring features of the Orion.
 
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LVLAaron

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To decide, start by defining what you need your system to do. Do you need a 400ah battery that can provide 100amps? Or do you need a 15kwh system that can provide hundreds of amps?

I'd use internal BMS's for a battery that's up to a couple hundred amp hours...

If you're building a large stationary system, use an external one.
 

Rocketman

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I use a Batrium Core - an external one that can also talk to Victron. The external ones seem much more versatile. Mine has like seven relays that I can use to control stuff. Heating. Cooling, charging, discharging, alarms, contactor, breakers, etc… Plus some input lines as well.

It can also be setup to run any voltage and cell configuration. But they do cost more.

In my MotorHome I wanted the ability to keep the boost mode so I could start the big diesel motor with the house batteries as 12v. The cells could handle that, but I needed a bms that could - so external became the choice for me.

Everyone has different needs and wants - the real trick is finding one that best satisfies you.

Good luck
 

brendanisatsea

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An external BMS uses a contactor and an external current measuring device. My Orion has a heritage from EVs. It can give a fairly good measure of SOC by counting Coulombs. It can provide additional signaling to put a motor controller in turtle mode when SOC is low. It is more expensive than the internal BMSs. I have also built some 12 volt packs and used an Overkill BMS, but for a 42 kWh pack, I want the comfort and remote monitoring features of the Orion.
That explains it nicely, so does this mean an external BMS will save your battery from overcharge/discharge via contactor, while an internal BMS on the other hand saves your battery by cutting off the neg terminal? And in terms of their Amp rating, an external BMS capacity is defined by the contactor/relay, whereas the internal BMS is defined by it's own Amp rating.... am I understanding it right?
 

Rocketman

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Yes you have correct - the contactor or cells are the limiting factor.

Here is my thread on the battery I made with a Batrium BMS. If you are interested.

 

Ampster

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And in terms of their Amp rating, an external BMS capacity is defined by the contactor/relay, whereas the internal BMS is defined by it's own Amp rating.... am I understanding it right?
The contactor is purchased separately so there is no practical limit to an external BMS. All the OEM EV manufacturers use external BMSs with contactors.
 
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Marinepower

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I did a LFP installation last year on my sail boat and went the "external" route. BMS I used is the Electrodacus.

External is the way to go if you have large induction or capacitive loads like a windlass, electric winch, etc which you plan to put on your LFP bank. REC bms seems like the gold standard for marine use, if you are going with a 12 volt set up.

However, it's a lot of extra expense and complexity to do an external properly.

If I could do it all again, I would put all my large intermittent loads on a pair of lead acid start batteries and all the small but constant daily consumer loads (fridge, lights, instruments, stereo, fans, etc ) on the LFP and use an internal BMS ( ideally with split charge and discharge bus). The one exception would be the inverter, which I would attach directly to the LFP bank (by-passing the BMS) but use the inverter's remote on/off to control it for LVC via the BMS (Will P has a video on how to do this - unlike Will, I would add a class-t fuse, a pre-charge resistor and a shut off switch however)

I know this was not really your question - but those are some reflections on the External BMS having just recently gone down that route on a boat....

MP
 

brendanisatsea

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I did a LFP installation last year on my sail boat and went the "external" route. BMS I used is the Electrodacus.

External is the way to go if you have large induction or capacitive loads like a windlass, electric winch, etc which you plan to put on your LFP bank. REC bms seems like the gold standard for marine use, if you are going with a 12 volt set up.

However, it's a lot of extra expense and complexity to do an external properly.

If I could do it all again, I would put all my large intermittent loads on a pair of lead acid start batteries and all the small but constant daily consumer loads (fridge, lights, instruments, stereo, fans, etc ) on the LFP and use an internal BMS ( ideally with split charge and discharge bus). The one exception would be the inverter, which I would attach directly to the LFP bank (by-passing the BMS) but use the inverter's remote on/off to control it for LVC via the BMS (Will P has a video on how to do this - unlike Will, I would add a class-t fuse, a pre-charge resistor and a shut off switch however)

I know this was not really your question - but those are some reflections on the External BMS having just recently gone down that route on a boat....

MP
It wasn't really my question, but you definitely hit the nail on the head - My plan is to accommodate a few larger loads like power tools on a 2000W Victron Multiplus, and a Lofrans Tigres windlass that can consume up to 100A. My design keeps a lead acid starter battery. Considering I only use my windlass when the engine is running (i struggle to think of a circumstance where the contrary would take place), maybe it's possible to keep the windlass wired directly to the "starter" battery like you've indicated.

I'm curious, is it just general complexities in installation/maintenance/operation of your external BMS, or was there something else that has you reconsidering your decision? Also, what's the benefit of your inverter bypassing a BMS?
 

Marinepower

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It wasn't really my question, but you definitely hit the nail on the head - My plan is to accommodate a few larger loads like power tools on a 2000W Victron Multiplus, and a Lofrans Tigres windlass that can consume up to 100A. My design keeps a lead acid starter battery. Considering I only use my windlass when the engine is running (i struggle to think of a circumstance where the contrary would take place), maybe it's possible to keep the windlass wired directly to the "starter" battery like you've indicated.

I'm curious, is it just general complexities in installation/maintenance/operation of your external BMS, or was there something else that has you reconsidering your decision? Also, what's the benefit of your inverter bypassing a BMS?

There is nothing wrong with the External BSM route but it gets pricey and a bit complex. Some of that complexity in my case was self-inflicted.

I have 500amp latching relay for the load side of the LFP (BlueSea), 200amp solid state relay for the charge side (Victron) . That was $450 in relays.

Plus I have my BSM mounted on my electrical, panel which required fusing and extending all the sense wires.

My BMS also controls my external alternator's regulator, so it shuts down charging in case of a HVC event. Same with my solar charge controller and an LVC to my inverter's remote shutdown.

I also moved my batteries location out of the bilge to under my nav seat, to keep the system high, dry and away from engine's heat and vibration. It was a lot of wiring running though the boat to accomplish all of this.

An Internal BMS and DC-DC charger solution would have been be a lot simpler. But if you don't have a lot of solar, or run your engine a lot, you may not get the recharging power you need from the DC-DC charger. I have limited solar, so I struggle to get my daily power usage back into the batteries the next day. Part of the reason I went for the system I did.

See pic below of part of my Frankenstein system.

IMG_7338.jpg


Re. the inverter, I'm sure many other will disagree, but an inverter load on internal bms will put a lot of strain on the internal FETs. When they fail, they can fail closed. Unless you live aboard, on a boat will be turning your inverter on and off. There is a huge inrush of current as the invterer's capacitors fill. Basically a dead short for a few milliseconds. If you do this with an internal BMS, you may fry the FETs. If you pre-charge the inverter, no problem, but unless this is automated you are going to forget one day. Perhaps better to wire directly to the LFP (with its own, fuse. switch and a manual pre-charge resistor) and let BMS signal the Victron's remote ON/OFF function if there is a LVC problem. As I said, many will likely disagree with this strategy.
 
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