I have a boat house bank of three 24v Victron NG batteries (900ah bank). They are connected to Class T Power-In’s, a Lynx BMS NG and Lynx Distributors. My inverter/charger is a Victron Multiplus 120v AC, 24v DC, 3000W and all is controlled via a Cerbo GX.
If I disconnect the house bank by opening the contactor on the BMS then try to turn the BMS contactor back on I get a “#35 Precharge Timeout” error. Repeatedly. I have read about the possible reasons and tried various ways to get it to connect.
The really bad thing is that every time the BMS re-tries to connect, when it is precharging it passes voltage back to our main breaker panel, but in various voltages like 12v, 18v, 22v etc and then fails again via timeout. That seems like a dangerous condition and a potential fault of the BMS. I don’t believe the precharge circuitry should be allowing that voltage through until the contactor closes and you can get a clean, full battery bank voltage.
The only way to get the BMS to close again is to turn the Multiplus completely off, then on again and turn the BMS on at nearly the exact same time the Multiplus kicks in to charger mode. Then the BMS apparently sees that full charge voltage and behaves.
Any ideas as to what this means and how to solve it?
The precharge unit is to precharge the capacitors in the Multiplus so that when the Contactor closes it does not spark and erode the contacts, you can not precharge after the contactor has closed.
The precharge timeout suggests you either have other capacitive loads on the system being charged up or you have other devices taking some of the precharge current. Make sure all loads are turned off and the Multiplus is off so the only thing the precharge is doing is charging the Multiplus capacitors.
So is the problem primarily resistive or capacitive loads that cause this issue? We had disconnected virtually all the loads except for a couple small resistive ones (IP camera etc)
And on a large boat how is this expected to work in the real world. There are often loads that are unacceptable to shutoff and/or devices that don’t tolerate restarts well and may be connected directly to the batteries ahead of the BMS. And the Cerbo GX is connected to the BMS upstream of the contactor as per Victron specs of course.
Getting that contactor closed without a lot of fuss is critical on a boat and can be a matter of life safety. I can’t imagine Victron made the BMS precharge so sensitive that it creates this problem all the time. How can I determine if maybe my BMS is faulty?
All I can reiterate is the precharge current is limited by a resistor so it has a finite value. It has to precharge the capacitive load to a set voltage in a set time. If some other parasitic loads take some of that current it can not reach the required voltage. Loads upstream.of the BMS have no bearing on this. Can you isolate the Multiplus and try.
On the older Lynx BMS you could take the cover off the BMS you should see the large wire wound resistor and could check that to see if it is blown. I do not know about the NG version, I can not remember what mine looked like and I am not on the boat at the mo. You seem to be getting some voltage through. Talk to your dealer and ask about this. Have you the 500A or 100A version.
Thank you Justin - that’s a good point and we in fact do not have any devices connected ahead of the BMS. The Cerbo is connected to the BMS’s Aux out, but that is by design.
I’ve hit this same problem with the Lynx BMS 500 (the non-NG version). Had to yank half a dozen fuses to get the system to fully power on and make it past this. Super annoying - some of these loads I never designed a manual switch as they are always on devices, sorta like the Cerbo. None of them in total are huge loads.
Yes, that is the problem on a large boat. There are parasitic loads or hard wired devices that don’t all run conveniently through the electrical panel. These may not be large loads, but apparently enough to cause these timeout issues. I got around my problem by turning on a Mastervolt charger which provided enough current to apparently make the BMS happy enough to engage again, but this seems like it should be unnecessary.
Experts: Is there perhaps some specification of specific load and or resistance figures within which the BMS will behave as intended? I ‘think’ I recall seeing a spec that indicated that resistance of below 20 ohms would cause a precharge timeout, for instance. Any additional info?
Thank you pwfarnell, I have seen that resistance figure in the datasheet. And maybe I’m misunderstanding something, but what about a spec for allowable actual load to restart the BMS? For a hypothetical example, if there is a 15A load seen during precharging is that unacceptable and will result in a timeout, but a 3A load would not? (My system is 24v and my Lynx BMS is the 500A NG version).
I may be over simplifying this because I do not have access to Victron info. I read this as a nominal 24V system at lets say 26V and a minimum allowable resistance of 20 Ohm is a maximum allowable load of 1.3A.
Thank you pwfarnell.
You’re right that V/R=A. So in my case 1.3A.
As I previously wrote, on a boat or larger RV it is very easy to have some loads that may be fused as opposed to running through the main breaker panel. These could EASILY exceed 1.3A. As a practical matter then, one would have to not only turn off all loads at the panel, but then crawl through the boat disconnecting all those fused loads as well in order to restart the BMS? Or alternatively, fire up a generator in order to power up a charger or power supply onto the system to eliminate that load?
Those seem like rather heroic measures to have to take just to re-engage one’s critical main battery disconnect especially under duress perhaps while at sea. Page 30 of the manual seems to indicate being able to force the BMS out of ‘off’ mode by powering the BMS off and then back on - but that does not solve the timout. Are there other ways to force the contactor closed?
If all the above is true I’m feel like Victron engineering may have let ‘perfect be the enemy of good’ here and potentially created an unsafe situation for boaters. Could this be fixed in firmware?
I can’t answer the questions you raise, those of us moderating the site are not Victron employees but installers or enthusiasts.
A possibility is a precharge circuit from the batteries bypassing the BMS through a push to make switch so you can not inadvertently leave it activated. Or could you precharge from engine start battery if the same voltage and have a common negative. I know it is a work around rather than a solution.
It depends on what the resistance is to know if it would be useful, if it is the same resistance as in the Lynx it may still be too low a current compared to your loads at start up.
