Fusing control circuits -vs- power supply circuits

Scott SV Tengah

There are two separate issues here:

The first thing I spec'd when I installed my Victron lifepo4 was 3 mega fuses, one per battery pair. Each fuse is big enough to run the bow thruster on its own, in case the other fuses happen to blow. That protects the wiring in case you have a short. The amount of current that a lithium battery can deliver is a blessing if it's used properly and a curse if it's going through an unintended short.

The BatteryProtect or low cell voltage disconnect is there to protect the batteries (hence the name Victron gave it). The largest one they make is 220amps, called the BP220. The bow thruster, hydraulic passerelle, a 5kw inverter running multiple appliances will easily fry it. My BP220 lasted for less than an hour after installing it. First time I touched the passerrelle remote control, the BP220 melted.

After using lithium for 4 years now, even if Victron or someone else made a 1000amp BatteryProtect, would not install it as I still do not think a blunt load disconnect of all uses is advisable. I would not want to lose nav lights and autopilot just because I didn't top balance for a few months and despite being at 25% SOC, one of my cells dropped below 2.8v and triggered a low voltage disconnect. Or if I'm away from the boat and the bilge pump is running due to a leaking raw water hose with the aforementioned imbalanced batteries, I would rather the pump keep going and kill one of my admittedly expensive batteries versus have the engine room continue to fill with water.

2007 A54 #69
SV Tengah

Dominique Sery <dominiquesery@...>

Hello David,

I  totally agree with you.  The Victron Ve-BMS is not a protection for short circuits or overcurrents.  Fuses of the appropriate size are installed as close as possible to each 24 volt battery as well as on the devices according to Victron's recommendations.  In addition, most circuits have a circuit breaker.

Kind regards

David Vogel

Hi Dominique,


To reinforce what Olivier says, I am not sure of the wisdom of protecting only the control-side circuits – as, an uncommanded activation of the electrical motors, such as we would see in the case of a short circuit or stuck contact in the control relays, will see the full current being drawn from the lithium batteries – even when the control switch is not activated, and even if the control circuit is disconnected.


To properly protect the (lithium) batteries (and the intervening wires between the battery and load), requires fusing (or circuit breaker, or other protection device) on the main supply cables, located as close to the battery as possible.  AMEL elected not to fuse the power cables to the Bowthruster, and some (many?) owners have seen this as a significant shortcoming and installed an appropriate fuse on this circuit.  I believe that the risk of failure with catastrophic consequences is increased with lithium batteries that are not fully protected on the load side.


Please, I am more than happy to be corrected on this, and learn, if I am wrong.


With kind regards to you,



SM#396, Perigee



From: <main@AmelYachtOwners.groups.io> on behalf of "Dominique Sery via groups.io" <dominiquesery@...>
Reply to: <main@AmelYachtOwners.groups.io>
Date: Monday, 15 August 2022 at 8:55 pm
To: <main@AmelYachtOwners.groups.io>
Subject: Re: [AmelYachtOwners] LiFePo4 batteries: STERLING (with B2B charger) -vs- VITRON


Hello Olivier,

yes, it works fine on my boat. The « cockpit controls » shut down if the Ve bms voltage is disconnected. I tried it. With the Ve-bms on, every thing works. If i disconnect the signal from the Ve-bms all the big loads are off (bow thruster , windlasses , electric winches , all the motors that needs the « command cockpit » on at the 24v panel).
i use also a battery protect 65 to drive my alternator (24 v Mastervolt)
best regards

Mon anglais n’étant pas fameux, je vous réponds également en français pour éviter toute confusion.
Bonjour Olivier, je vous assure que ça fonctionne bien. Le Battery protect est activé par un courant 24v qui vient du Ve-bms Victron. Tous les disjoncteurs du tableau 24 v sont opérationnels. Si le Ve-bms se met en alerte de tension d’une cellule de batterie basse (ou de température) il désactive le courant qui active le BP 220, immédiatement tout le tableau 24 volts est éteint, y compris le disjoncteur « commandes cockpit » alors les gros consommateurs voient leur alimentation éteinte et ne fonctionnent plus. Je viens de refaire un essai à l’instant, le propulseur et les winchs en cours de fonctionnement s’arrêtent immédiatement dès que le BP 220 ne reçoit plus la commande du Ve-bms,
J’utilise le même système pour contrôler mon alternateur Mastervolt 24 avec un smart BP 65 qui me permet en plus d’arrêter mon alternateur avec mon téléphone (une alarme sonore sur le BMV 712 à 27,6 v 95% de charge) lors des navigations au moteur prolongées.
Amicalement à vous

From: <main@AmelYachtOwners.groups.io> on behalf of "Olivier Beaute via groups.io" <atlanticyachtsurvey@...>
Reply to: <main@AmelYachtOwners.groups.io>
Date: Monday, 15 August 2022 at 8:12 pm
To: "main@amelyachtowners.groups.io" <main@amelyachtowners.groups.io>, "main@AmelYachtOwners.groups.io" <main@AmelYachtOwners.groups.io>
Subject: Re: [AmelYachtOwners] LiFePo4 batteries: STERLING (with B2B charger) -vs- VITRON


Hello Dominique,


Your Victron BP220, as it is connected on your pictures, does not protect the batteries against the big loads/amperage. The bus where it is connected feeds only small loads (lights, fridges, solenoids for big loads, autopilot and all the items connected to the galley 24V panel).

If you want to protect your batteries from the big loads (bow-thruster, winches, furlers, windlass, etc...), you need to install a battery protector at the source, which means close to the battery bank and using the big 24V positive cable. 

And if you do that with your BP 220,  you will see that it will blow because of the 500amps of the bow-thruster...




From: <main@AmelYachtOwners.groups.io> on behalf of "Dominique Sery via groups.io" <dominiquesery@...>
Reply to: <main@AmelYachtOwners.groups.io>
Date: Monday, 15 August 2022 at 5:32 pm
To: <main@AmelYachtOwners.groups.io>
Subject: Re: [AmelYachtOwners] LiFePo4 batteries: STERLING (with B2B charger) -vs- VITRON


Hello Scott, 

I confirm that there is no problem with the battery protect 220. I use it as a protection for the whole 24 volts circuit, including the bowthruster (480 A), the windlasses, the electric winches...

I installed it in the cabinet above the sink. I put the 24 volt inlet on the Battery Protect inlet and the outlet on the 24 v panel inlet. The BP is controlled by the Ve-BMS. 

It works perfectly in my boat, it's the relays of the controls of the big consumers that are activated/deactivated so low consumption.

I think that the BP 220 is overdimensioned.

I had bought it thinking to install it near the batteries but it is not possible, too low power.

I hope this will help you.


Some pictures