# Multiplus inverter low battery Alarm as the power demand increases

Hello [Victron Staff] and @Alexandra ,

I recently installed a backup system consisting of a Multiplus 48-5000/70-100 with 12 Gel batteries of 200Ah each, for a total capacity of 600Ah-48V, the multiplus automatically starts a 13KVA generator when the load exceeds 3500W, and the customer wants to reduce its fuel consumption by increasing the power of the inverter.

The problem is that I noticed that the customer's load very often goes from 2600 to 7000W when he turns on the microwave, as well as all his air conditioners, even if it lasts only a few minutes, and this produces a strong drop voltage in the batteries (High DC ripple)leading to the starting of the genset.

The section of the cables between the inverter and the batteries had been well respected during the installation, this means that the voltage drop cannot be caused by an insufficient cable cross-section and leads me to believe that the batteries are small compared to the load.

What formula can I use to calculate the current drawn from the batteries when the load is high(7500W), and thus determine the optimal capacity of the batteries?

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@Deutch

Watts =Volts x Amps. So to calculate the draw Watts / (battery)Volts = Amp draw from the bank.

C rating of the Gels comes into play though.

How is your bank wired? https://www.victronenergy.com/upload/documents/Wiring-Unlimited-EN.pdf provides some useful information. DC ripple other than you bank being undersized can also be cause by high resistance connections. This can be caused by the way the bank is configured as well as cable sizing and length. The manual shows what cable size is necessary.

7000W is an overload for the system in any-case. At 48v that's easily 145A out the bank. You are also most likely exceeding the C rating of the batteries they are usually c10 or c20 so at most you should only be pulling 60A out of the bank at a time (2880W). You would need some clever wiring (check out Victron Wiring unlimited) and at least double to bank to match the max rating of the inverter without overload. Triple to make the overload scenario described above. Basically you are running into the shortcomings of the battery chemistry.

1625597367902.png (32.6 KiB)

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Hello @Alexandra ,

What can I do to apply a C5 regime to the batteries?knowing that the overload only lasts a few minutes when the microwave and the creamer are switched on?at C5 the current drawn from the batteries is 120A(600/5) and this represents a power of 5760W, without increasing the capacity of the batteries but replacing the Multiplus 48/5000 by a Quattro of 48/8000VA, can I draw a power of 5760W from the batteries without the voltage of my batteries dropping considerably?

DC12-200 Ritar battery.pdf

The c5 rating is the capacity of the bank drawn out in 5 hours. The ritar shows on the datasheet now you do not calculate the bank size at 200AH but at 172AH. So it is now smaller.

Again if you have bad sulphation in the batteries the capacity and strength are even lower.

Hello @Alexandra ,LPC12-200.pdf or everyone can I help me,

Please, look at the data sheet of this battery, if I understand well what @Alexandra explained to me, with a power demand of 7500W, which represents a discharge current of 156,25A under 48V, my battery voltage should not drop considerably as it is the case today, because it represents a discharge regime of 3h C3, with a cell tension of 1,75V, which makes a tension of 42V for a system of 48V.

Would the problem come from the section or the length of my cables? If I put a more powerful inverter for example a Quattro 48V-10000VA/140, I revise the section of the cables, but do not increase the capacity of the batteries because the peak of power lasts only a few minutes, will my batteries supply this current without the voltage of the batteries dropping considerably, and thus the alarm of low battery does not light up, and the system does not start the genset because of the low battery?

lpc12-200.pdf (289.8 KiB)
@Deutch

Consider switching to lithium.

Lead based systems have a terrible lifespan and these issues you are seeing always occur unless you have a massive bank cleverly wired for good current sharing. (Increasing install costs) they have a high internal resistance which only gets worse during use.

They don't like high discharge currents (despite what the datsheets say) and building 48v banks with 12v batteries comes with its own headaches.(https://www.victronenergy.com/batteries/battery-balancer). What happens is batteries need to be balanaced in the bank and bank maintenance needs to happen. You need to make sure they never discharge past 50% and always charge up again and have the full absorption cycle to reverse the chemistry. It becomes a nightmare when offgrid.

Battery rotation and checking for over and undercharging as well as a host of issues by the user or installer

It is easier both as an installer and the user to have a 48v battery with a good bms that allows the current draw applicable for the inverter draw.

From personal experience as an end user, I will never go back to a lead based battery. The cost per cycle is lower and the lifespan is longer. My lithium batteries have paid me back already less than 2 years of use.and there is plenty of life still left. In a system where they cycle and have heavy loads lead acid are never a good choice as they sulphate (the killer of lead acid batteries) and become less and less useful and able.

Hello @Alexandra ,

Thank you for this answer, you have just opened my mind, I understood that there is a maximum current not to draw from the batteries and I think that's the problem, because the section of the cables between batteries and inverter had been respected (2x70mm) and all batteries are connected to the DC bus barre before entering the inverter.

I think that the current drawn from the batteries is higher than the current to be drawn from the batteries because when there is a high load the voltage of the batteries drops approximately to 36,5V and the inverter shuts down immediately.

I understand that even if I replace the existing inverter with a Quattro of 10000VA, the problem will not be solved because of the current drawn from the batteries, I have to make sure that the batteries will be able to accept to draw a power of 7000W, which represents a current drawn from the batteries of 146A.

Thank you for your answer @Alexandra,thank you very much I am slowly beginning to understand where the problem comes from.

1 comment

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7000W at 38v is now 189A. As the bank voltage collapses the amp draw becomes higher. It is a vicous cycle.

The other possibility is (if you see how to wire a bank in the book linked) you have one or two weaker batteries and their voltage is collapsing faster than the rest of the bank and so the whole bank is brought down as the rest of the batteries are working harder

In short the bank as a whole that is there may not be healthy anymore.

hello!

@Deutch, consider using lithium battery chemistry. lithium batteries have very very small internal resistance, thus they are able to provide super high currents compared to gel, agm or slea... depending where are you located, probably you can find second hand electric vehicle battery modules. by far they are the best option, and they are designed especially to provide extreme peak currents.

for example, i've bought 4 tesla modules for my camper, each 7s (26v, 100a), so i have 26v 400a bank, which easily can provide more than 400a. one module was 450 euro, so you can make the math... the tesla modules have a big advantage that they are using lots of 18650 cells in paralel, which keep balanced each other, so they can not really go unbalanced. and the modules already come with the original bms. you can hack an esp or arduino on them and is ready to go ;)

https://github.com/collin80/TeslaBMS

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