Voltage drop from inverter load (large load)

samhall55

Hi all,

I have recently just completed a van conversion with my home made system of which I did loads or research and copied others systems.

That being said I would only just call myself competent and I already have problems hence I have found myself here...

I have:
2x 140ah batteries - https://www.tayna.co.uk/leisure-batteries/varta/lfd180/ 
1x 2000w inverter

When I try and use my coffee machine (900w 240v) via the inverter, the battery voltage very quickly (10 seconds) drops to 11.4v and many alarms start (battery monitor and inverter).

I was under the impression that my batter capacity would be enough for this application? 

any help would be greatly appreciated! 

samhall55

I should have said - when using the coffee machine my battery monitor tells me that I am pulling around 90amps from the batteries...

Photowhit

samhall55 said:

I should have said - when using the coffee machine my battery monitor tells me that I am pulling around 90amps from the batteries...

90 amps is abut right at 11.4 volts that's 90x11.4=1026 watts so the 900 watts of load + the inverter efficency 85-90% of converting Dc to AC.

The normal 'problem' with voltage drop is too small a wire connecting the batteries too the inverter.  That would be the first thing to check.  Enter your wire gauge into a voltage drop calculator. Here's one;

Voltage Drop Calculator

Also check your connections, no alligator clips, everything tightened down!
Be sure the battery is connected directly too the inverter. NOT through the load side of a charge controller.
Be sure your battery is near fully charged! This is a large load so you will want your batteries too be pretty well charged to take on a large load.

At 90 amps the rating of the battery bank will be smaller. There may be more information about this at the battery manufacturer. But 90 amps from a 360 amp battery bank is 1/4 it's capacity, the battery is rated for discharge at 1/20th of it's capacity. Most AGMs can handle this reasonably for a short time, but look for a capacity for this discharge rate. Which is perhaps what you did when you say they are 140 ah batteries?

Of course another would be to find a lower wattage kettle... but trouble shoot your system... See if there is a max discharge rate for the batteries.

samhall55

Thanks, thats really useful info.

To be fair, the cables from the main battery connections to the inverter looked small/undersized to me but it is what was supplied with the Renogy inverter so I went with it - not wanting to mess anything up by using different wire size to what was supplied by the manufacturer (albeit bigger...).

My battery monitor is a Renogy shunt that measures Volts across the battery bank and amps in and out. are you saying that undersized wires somewhere along the system can cause a voltage drop measured across the batteries? (Please excuse me if that is a stupid question...).

Marc Kurth

You appear to be using a flooded dual purpose marine/RV type battery, as opposed to a true deep cycle battery. From the link, it does not appear to have accessible fill caps?

Assuming adequate wire sizing, a fully charged battery bank of 2 x 180 ah, you should be able to maintain higher than 11.4v with a 90 amp load for longer than 10 seconds.

Photowhit

samhall55 said:

To be fair, the cables from the main battery connections to the inverter looked small/undersized to me but it is what was supplied with the Renogy inverter so I went with it - not wanting to mess anything up by using different wire size to what was supplied by the manufacturer (albeit bigger...).

My battery monitor is a Renogy shunt that measures Volts across the battery bank and amps in and out. are you saying that undersized wires somewhere along the system can cause a voltage drop measured across the batteries? (Please excuse me if that is a stupid question...).

So it looks like 4 agw 3' cable is supplied, even over just 3 feet there's a 1.4% voltage drop. So long as you are using the supplied cables and didn't goo and buy 12 feet of 4 agw (5.5% drop) you are likely fine. 

Was your battery bank fully charged when you did your experiment? If/when you try it again, you might record the voltage before you start on the battery at rest (No load or charging for 4 hours), then see how long and to what level the battery recovers over the minutes and hour after.

You are in Europe or UK? If soo yu might have Lead Selenium batteries, which we are less familiar with in the US. I assumed they were AGM (Absorbed Glass Mat) batteries, which hold the electrolyte in a substrate and becomes 'maintenance free', but these batteries are very sensitive to over charging. Once over charged they may have a very minimal capacity but show good voltage.

Varta use to be part of Johnson Controls which is/was a huge battery manufacturer.

samhall55

ok, all understood, the batteries do not have fill caps (I assumed that defines them as maintenance free). 

I have tried to use the coffee machine first thing in the morning - Battery monitor say the state of charge is around 96% with 12.8v(ish). this goes very badly and once everything shut down and the battery monitor reset itself (I have read that this may have 'Flat lined' the batteries??). More success in the middle of the day when my 500w of solar is putting in 20amps - I still get low voltage and alarms but the inverter still works and I end up with a successful coffee!

Correct that I am based in the UK, currently on holiday, living in the camper van in Germany.

Many thanks for you help and comments.

BB.

For a flooded cell lead acid battery bank with 2x 12 volt @ 180 AH or 12 volts @ 360 AH bank.... Suggest the maximum continuous load would be around (basically 250 Watt AC inverter max capacity per 100 AH @ 12 volt battery bank):

• 250 Watt AC inverter capacity * 360 AH * 1/100 AH = 900 Watts

So--A 2,000 Watt inverter is "over-sized" for this battery bank--Pulling 900 Watts should be OK from a full charged/well maintained battery bank.

At this point--I think you may be having "battery issues" (assuming you have short/heavy cabling, use bolted clamps and not "alligator clamps"--And you are measuring voltage at the battery bank terminals and seeing the voltage "diving" at those terminals. (and you it does work "better" with 500 Watts of solar input)

First question, did these batteries set on the shelf for >1 month without charging (early sulfation of batteries--Loss of capacity)

Second question, what is the charging voltage you are seeing (and charger set point) for charging... Nominally, 14.4 volts or so and once 14.4 volts is reached, hold for 2-6+ hours to fully recharge (shallow discharge ~2 hours... Deeply discharged ~6 hours absorb time).

Not a deeply technical "manual" for Varta--But some details on charging (I think yours is flooded cell type). Note: Our "standard" for for recharging Lead Acid batteries is to recharge if 75% or lower state of charge (to prevent rapid sulfation during non-use)

-Bill (below is from Varta):

PS: I fixed a typo (I think) from Varta below--- 0/1 V should be 0.1 V

https://www.varta-automotive.com/bg-bg/Поддръжка/charging/how-to-charge-a-car-battery

Instructions on charging a battery safely

• Fully recharge the battery as soon as possible following each discharge
• Always check that battery charger is suitable for the battery
• Never use a battery charger without automatic shutoff
• An electronic voltage-controlled battery charger, preferably based on an IUoU charge profile, is recommended
• Charging to avoid acid stratification (not needed for AGM)
• Use a charger with IU characteristics and higher charging voltage (2.6 V/cell)
• This overcharge should only be used for a short time period to avoid water loss
• The open-circuit voltage after charge should be 2.12 - 2.13 V/cell
• For every 0.1 V difference to 12.7 / 12.8 V, the battery should be charged for 1 hour
• Never charge frozen batteries or batteries with a temperature above 45˚
• Connect the positive battery terminal (+) with the positive charger terminal, and the negative battery terminal with the negative charger terminal
• Do not turn on the charger until the battery has been fully connected
• When charging is complete, first turn off the charger
• Stop charging if the battery becomes hot or acid escapes!
• When charging, ensure good ventilation

Test a battery

The best and easiest way to test a battery is using a voltmeter or multimeter to measure the voltage. Once you know the exact voltage, use the table below to gauge the state of charge.

Make sure the battery is charged

To ensure that the battery can supply its stated starting power, it first needs to be fully charged. The recommended charging current is 10% of nominal capacity in amperes (e.g. a 4 Ah battery requires a 0.4A [Ampere] charging current). We recommend that you ensure your battery is fully charged before fitment to ensure a long service life.

• A new battery after activation is approximately 80% charged.
• Initial charging is always recommended. Never quick charge.
• You can find a detailed description of how to charge the battery in the instruction manual supplied with your battery.

Marc Kurth

Bill, as always you provide a plethora of valuable and important information. My experience is narrow but very deep with a particular type of battery, and I will share some data specific to Concorde Sun Xtender, Lifeline, and Chairman branded AGM batteries.

It is important to understand that when using voltage as an indicator, the measurement must be taken 4+hours after any charging or discharging. The voltage measured at 4 hours will be the same as that measured at 3-4 weeks at 70F to 80F.

New, fully charged battery:

100% charge: 13.0v at 0F to 140F

50% charge: 12.2v

0% charge: 11.6v

Obviously, the numbers under load are vastly different than these "static" numbers. The BIG number that I teach my customers: 10.5V is 0% remaining capacity regardless of load between C/0.50 and C/100.

samhall55

Wow, thanks for all this info!

I have all of my electrical components and main wiring bolted to a wooden board that is then mounted on the wall in the back of the van.

yesterday I took the board off the wall and found that where I have all of my main Negative wires bolted together, the main negative wire to the batteries and also the negative wire to the inverter were melted and damaged.

It turns out that the bolt was not done up super tight resulting in a (only a little bit) loose connection.

I have replaced the wires, added some penny and spring washers and retightened the arrangement (with the addition of some copper grease). The results are that I am seeing a much more stable voltage on my battery monitor and now when I use the 900w coffee machine the voltage drops to 11.8v and holds steady.

Lessons learnt! Also I do plan to add another 180ah battery to the arrangment in the future to hopefully be able to take advantage of the full 2000w inverter.

Thank you all who contributed, its very valuable for someone like myself who is trying to navigate a new world of 12v electrics!

Cheers!

Photowhit

samhall55 said:

yesterday I took the board off the wall and found that where I have all of my main Negative wires bolted together, the main negative wire to the batteries and also the negative wire to the inverter were melted and damaged.

Glad you got it figured out!

In US home wiring we have a rule that al wiring connection must have access panels. It's good to be able too easily check those connections.

dixonge

Marc Kurth said:

You appear to be using a flooded dual purpose marine/RV type battery, as opposed to a true deep cycle battery. From the link, it does not appear to have accessible fill caps?

Assuming adequate wire sizing, a fully charged battery bank of 2 x 180 ah, you should be able to maintain higher than 11.4v with a 90 amp load for longer than 10 seconds.

At the link they provided it is showing 1050 CCA! Basically the UK version of hybrid RV/Marine batteries. Great for starting a generator or lights or water pumps. Probably not designed for inverters.

Glad you found the wiring issue before anything caught on fire!