question

thesee avatar image
thesee asked

BMV 700 battery capacity

I installed a Victron BMV 700 to my boat 12 months ago and it's really good. But:

When I read the on line manual it states to use the C20 battery capacity value.
There is no mention of this in the manual supplied with the unit.
I also have a Victron battery combiner (Cyrix-CT) connected between my starter battery and the house bank.

Battery details are: 4 x lead acid deep discharge 135 amp hour, C20 value 950 watt hours (C20 = 79 amp hours) house batteries.
1 x lead acid engine starter battery 100 amp hours no C20 figure available.

I have 100 watts of solar and wind charger to both starter and house batteries.
300 watts of solar to the house batteries.
300 watts of solar to the starter battery switched automatically by the battery combiner.

I have left all the other BMV 700 settings as standard. I just need to add the most accurate battery capacity figure.
BMV Battery Monitorinstallation
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4 Answers
Justin Cook avatar image
Justin Cook answered ·

@Thesee, what "online manual" are you referring to? The manual available online from Victron is literally just a digital version of the printed user manual included with your BMV-700.

From the (only) manual:

"3.3.1 About battery capacity and the rate of discharge: The capacity of a battery is rated in ampere-hours (Ah). For example, a lead acid battery that can deliver a current of 5A during 20 hours is rated at C20 = 100Ah (5 x 20 = 100). When the same 100Ah battery is discharged completely in two hours, it may only give C2 = 56Ah (because of the higher rate of discharge). The BMV takes this phenomenon into account with Peukert’s formula: see section 5.1."

Your battery as pictured is a 135Ah battery; I believe the discrepancy in the posted Wh rating vs the Ah rating is due to the manufacturer taking into account the maximum DOD of the battery; in which case, sure, in your 135Ah FLA battery you only have about 79Ah usable without killing the cells, but the capacity that you program into your BMV will still be the total rated capacity: 135Ah each so, if your 4 batteries are in parallel, 540Ah total. Your BMV will use the default Peukert 1.25% formula to determine your SOC based upon your rate of discharge, but it needs the actual Ah to be set properly as a basis for its calculations

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thesee avatar image thesee commented ·

Hi Justin

Thanks for that info, that's just what I needed and confirms my settings.

I did see a note in the online manual as I mentioned. Maybe I misread or misunderstood?

From your web site: Downloads, Manuals, BMV 700 700H 702 712

Top of page 5?

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Justin Cook avatar image Justin Cook ♦♦ thesee commented ·

Aha, I see it now; yes, you're correct, it does say that... never noticed before. And it's odd that your batteries give two different measurements... as @JohnC noted, typically the C20 rating is exactly the same as the Ah rating because C20 is literally how the Ah capacity is determined. In the case of your batteries, it looks like maybe the 135Ah is at C16 or something silly...

All that being said, still use your actual Ah rating on the batteries and let the BMV figure out what your actual usable capacity is; it'll adjust itself and its relative SOC after a few charge/discharge cycles.

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JohnC avatar image JohnC ♦ Justin Cook ♦♦ commented ·

Yeh, I found it, and you're pretty close. The Wh* is a 20hr rating down to the recommended (max?) depth of discharge. Never seen that done with pb's before - I should get out more..

https://cdn.tayna.com/datasheets/Exide-Marine-leaflet-2015-Europe-EN.pdf

I'd use 135Ah in the BMV, like Justin sez. If all paralleled as 12V, 4x = 540Ah.


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thesee avatar image thesee JohnC ♦ commented ·

OK thanks for your help. I thought the C20?WH rate was a little low compared to the 135 AH claimed. Anyway 540 capacity is in the BMV now and as you say it should calculate it’s own SOC after a few charges.

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sailingcal21 avatar image sailingcal21 commented ·

Thanks. Just what is was looking for. So the battery capacity setting is for the actual battery spec and the BMV will calculate the SOC at a 100 AH actual if the setting is 200 AH for FLA batteries?

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JohnC avatar image
JohnC answered ·

Hi Thesee. Your shunt should be connected to the -ve terminal of the batt(s) you're monitoring, with nothing bypassing it, including any connection to other batts. That determines the relevant bank.

Presumably that's your house bank. But you'll need to decide whether they're 135Ah or 79Ah. C20 is a common way of rating pb's, so where's the difference coming from?

That aside, if 12V batts in parallel, then 4x it when speccing the BMV.

Perhaps you could post us pics or maker's links to where you're deriving these Ah figures? C20 isn't something from the cloud, it means the same thing as Ah..

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thesee avatar image
thesee answered ·

Hi John

This is a pic of one battery, ther'e all the same (4) just not sure whether to use the full rated AH or the C20 rate for the battery capacity.

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thesee avatar image thesee commented ·

and now with the picture attached.

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danno avatar image
danno answered ·

Hi Everyone.
I also have a similar question and am seeking confirmation on what # to enter for Battery Capacity in the BMV 702 Setup. I just replaced my three 200 Ah lead acid batteries with three 255 Ah AGM batteries on my sailboat. One battery is on Battery Switch #1 and the 2 others are on Battery Switch #2, however I always have my Battery Switch set to “1 + 2”; meaning they are always combined together in parallel as one big house bank of 765 Ahrs.
I just set my BMV 702 to this number: 765 AmpHours. Is this correct?

Please let me know.
Cheers


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Justin Cook avatar image Justin Cook ♦♦ commented ·

@Danno, that is correct. Please note, however, that if you do ever switch over to just one of your battery banks, your BMV will no longer be reading accurately because you've now changed the Ah capacity of the bank. So long as your banks are always connected, however, and as long as the NEGs of both are correctly running through the BMV shunt and all loads and chargers are correctly running to the other side of the shunt, then you're golden.

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