question

Lionel SEYDOUX avatar image

Ah settings in BMV

The BMV 712 manual gives a C20 Ah setting as a rule. But in a self powered house, I never get under 80% soc in 24Hours which gives a C120 setting eventually C60 for peak load hours. On rare days I may be down to 70%SOC which gives C80 and C40 for peak hours.

Using C20 as advertised leads to SOC being overvalued and power intake being limited.

Should we not use C120 instead?

Lionel

BMV Battery Monitor
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5 Answers
Alexandra avatar image
Alexandra answered ·

@Lionel SEYDOUX

A battery manufacturer will set the C rating for your battery bank. The C rating is different to the capacity of the battery but is related to the capacity.

The capacity is normally calculated at 25°C over the amount of hours they expect you to discharge and charge at to reach their stated capacity.

If you discharge slower then in theory you are able to "use" more. But pull it faster and you will have "less" capacity as the internal efficiency changes.

The peukerts exponent helps with the charge efficiency calculations so that needs to be right for your battery type.

But in short if the manufacturer says it is 200AH set that as the battery capacity you should set.

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Lionel SEYDOUX avatar image
Lionel SEYDOUX answered ·

Thanks Alexandra, but:

C10, C20, C120? my manufacturer (Enersys) gives me several Cxx values: 1260Ah at C10 at 25° and 1714Ah at C120, 25°. Peukert only helps compute the capacity at other C values. That give me 1200Ah at C120, 20°

That's why I am wondering about using the C120 value as I only use 20% capacity in 24Hours (100% capacity in 120Hours)

Temperature: But in winter I am rather at 0°C (-4° to 5°C for the last 4 months) As far as I can see on the web (Sitelec) correction is about 80% for °C which would give a 1008 Ah capacity C120 at 0°.

SOC 100%: Presently VRM shows 100% as soon, or just before charging starts absorbing. I remember that absorbing covers about 20% of the total capacity. Therefore at that point between bulk and absorbing SOC should be shown as 80%, not 100%

Practical document: All that leave me confused! and I long for a document with practical guides, and without chemical or mathematical explanations whith are useless on board!

Cheers

Lionel


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That's why I am wondering about using the C120 value as I only use 20% capacity in 24Hours (100% capacity in 120Hours)

Is that discharge with a constant current?

Lionel SEYDOUX avatar image
Lionel SEYDOUX answered ·

I find on Internet a fairly practical document called "Thèse DELAILLE" . Many theoretical element but many practical ones too!

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Lionel SEYDOUX avatar image
Lionel SEYDOUX answered ·

Seb71: Tks..

No its not a constant intensity: my Multiplus is on (happy to have something to eat if I climb up in 4feet of snow). From what I see on VRM its between 0 and -3.5Amps.

Is there any acid level tester with WiFi so I can follow that through VRM? Id rather doubt it,but that would be a good idea!

The great advantage of these very cold days is that they come with a bright sun and keep these bloody batts at a fairly good level

Any way Good week-end

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C rates are based on the current you discharge with.

gro avatar image
gro answered ·
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