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Michael Riley avatar image
Michael Riley asked

Powering VenusGX-VEBus BMS-Multi-MPPT-BP220 on Lithium

System on sailing boat: 2 x 200AH LiPo, VenusGX, VE.Bus BMS, 2xBP220, Multi 24/5000,, MPPT150/70, Lynx Shunt 1000 +Pwr in and Distributor, Digital remote. Shore Power / Generator manually selected to Multi.

Question: How to wire the +ve for Venus and MPPT whilst:

a. maintaining all loads after BP220 (except BMS) and Multi (as per doco);

Have read manuals for VenusGX, BP220, VE.Bus BMS, etc. Noted the warnings about a 'hang up' in case of BP disconnect due to low cell voltage and am therefore looking for a reliable 'restart' after a BP220 disconnect for low cell voltage assuming that our 'recovery' method is to say, start the AC generator which will power the Multi (Charger) and thus the Battery voltage rises.

As I understand it from wiring i.a.w. documentation the BMS load disconnect is connected to the Terminal H of BP (0.6 mA x 2 draw). Venus (110 mA @ 24V) is powered from BMS (Load Disconnect of BMS max 2mA). Multi, MPPT and BP220(2) are all connected to Lynx Distributor (i.e. after the Lynx Shunt). From this I understand that ALL loads (except BMS) are able to be disconnected on 'alarm' (Hi, Low, temp). From GX manual I understand that the VE.Bus BMS will wait for the Venus to power up and with above (Venus powered from Load disconnect of BMS - as per documentation) when will the Venus get power? Suggestion with power sequencing explanation please.

After the above, add in where the MPPT should be powered. Before the BP220 and I now have an uncontrolled load on the LiPo (i.e. it is night and not charging). After the BP220 and solar energy will not charge the LiPo until the Venus is back on line.

MPPT ControllersBMSBattery Protectwiring
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2 Answers
boekel avatar image
boekel answered ·

Short - partial - answer (for better answers you'll have to provide a diagram of how things are connected, I assume you have one?)

-inverters don't go through a battery protect, but connect directly to the batteries (through fuses etc.)
-you -don't- want to reach voltages where the bms shuts off the batteries, this is only for emergencies. during normal use you set the system to shut down well before the bms has to shut down the system, so start generator before 0% SOC etc.

-you do want to use a battery protect for DC users, so when you have something that draws the battery down when no solar / shore / generator is available you don't damage the batteries.

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Michael Riley avatar image Michael Riley commented ·

@boekel Thanks. Got all that and I am discussing the 'emergency' situation as a worst case - as you said it is a partial answer - missing is the answer to the question of where does the +ve of Venus and +ve of MPPT terminate? Also as you say in the last paragraph I have 2 x 200 A Battery Protects (BP220) do do exactly as you suggest - disconnect all those DC loads that are not also connected to the BMS via another means (i.e. network control). Currently the Venus +ve is being wired to the Load Disconnect of the BMS as per Venus manual/documentation. MPPT is wired on the 'disconnected' side of the BP220's so this means that I CAN totally remove the load from the Battery(s) and CANNOT therefore rely on solar charge bringing my LiPo back up after a low cell BMS disconnect. Will use the generator for that (started by its own AGM). Should test / power up today so was trying to get all the wiring tied off and labelled before this above question changed the layout.

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Michael Riley avatar image
Michael Riley answered ·

It is now almost a month since the last post and no answer forthcoming. For anyone seeing this post here is my (now) current understanding/thinking on the above.

a. Any battery has an internal discharge rate therefore ultimately the battery will get damaged due to low voltage. The issue is therefore more critically, 'how long' before this happens but even more importantly WHAT is to be done about it before this occurs.

b. If you have a Li battery then it is my understanding that you should have a BMS and a 'load-disconnect' mechanism (i.e. Battery Protect). These however only buy you time if there is no replenishment; thus the resultant outcome for the above is whether my 'intervention' (timely action) is 'automatic' or manual meaning:

- Do I believe that by leaving the MPPT connected to the 'Lynx Distributor' before the Battery Protect allows the opportunity for the next 'day' solar contribution to bring the battery voltage up;

OR - do I believe that this will not happen (i.e. fault) and therefore better to have the MPPT on the 'disconnected' side so that I extend the time before the voltage drop becomes non-reversible.

To answer that lets understand what the 150/70 MPPT draws without solar irradiance = approx 25mA (24 V) (not sure about Bluetooth version). At 1AH / 100AH rating of battery to determine remaining AH at low voltage disconnect we have 2 x 200 AH therefore 2 x (1x200) = 4 AH. The MPPT on its own would take 160 hours to deplete the remaining voltage (if that were to be where the damage occurred). In any case we are looking to have an input (no matter how small) within 12 hours (i.e. the next morning) so the MPPT will not 'materially' affect the damaging drain and could materially effect a saving solution.

Perhaps that is what @Boekel was meaning above by 'why would you' (put the MPPT after the disconnect. Well the answer is because the literature says that you should - the practical answer is that unless you believe a fault will occur which prevents the charging (hence the low voltage disconnect in the first place - plausible) then you are more likely to get a benefit from having the MPPT able to charge the battery(s) at the earliest opportunity. Finally - still have an 'operator' alarm to have a review' as to WHY the low voltage occurred in the first place at the earliest opportunity (no more than a couple of days would be my recommendation). Perhaps he still wants to have a diagram to indicate what is meant by before or after the BP220 (I 'presumeD' he can read either).

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