DC Isolation

[Bald Camper]

I am new to solar power. After checking multiple internet sites, the deseing of 2,000 watts for my travel trailer is almost completed. But I still have 2 questions:

When hocked up to shore power, le batteries will get DC from both the trailer and the solar. Do-I need some sort of isolation between the 2?

When travelling, le batteries will get DC from both the tow vehicle and the solar. Do-I need some sort of isolation between the 2?

Thank so much.

 

[CityGuy]

Assume you have a trailer?

 

[Bald Camper]

Yes a travel trailer

 

[Gary RV_Wizard]

Answer to both is no. Which ever source is delivering the highest voltage will dominate. Think of voltage as pressure - the weaker source can't push past the stronger one.

 

[Bald Camper]

Thank-you

 

[DonTom]

Which ever source is delivering the highest voltage will dominate. Think of voltage as pressure - the weaker source can't push past the stronger one.

You have two batteries. One measures 13 volts unloaded; the other measures 12 volts unloaded.

The one with 12 volts has a capacity of 50 amps. The one with 13 volts has a current capacity of a microamp.

You put both batteries in parallel, what voltage do you then have?

-Don- Reno, NV

 

[Contributor]

Since you are not putting a load on the batteries, then the answer is fairly simple. Current (electrons) flow to the higher potential so initially, electrons (negative charge are attracted to a positive charge) would flow from the 12V battery to the 13V battery. This results in positive current flowing from the 13V battery to the 12V battery resulting in a depletion of charge from the 13V battery until both batteries are at 12V.

 

[Great Horned Owl]

First of all, battery capacity is measured in Amp hours, not in amps. Stating a battery capacity in Amps is meaningless.

Secondly, you haven't provided near enough information to make any real calculations. At the very least the fully charged capacity, the internal resistance of both batteries, and the gauge and length of the connecting cables would be needed.

Thirdly., the voltage across the pair would be a function of time. Given enough initial information, it could be calculated by using differential equations. However that would assume that the second battery hadn't exploded and that the interconnecting cable hadn't melted. More likely than not, at least one of those assumptions would not be valid.

Finally, Amp is short foe Ampere. It was named for . Named for the 19th-century French physicist André-Marie Ampère. Therefore, it's a proper noun, and as such, should begin with an upper case "A."

 

[DonTom]

First of all, battery capacity is measured in Amp hours, not in amps.

AH is how long the charge will last at a current rate. The total amp capacity can be different than the AH when time is NOT a factor.

"Cranking amps" is not cranking AH!

-Don- Reno, NV

 

[DonTom]

Since you are not putting a load on the batteries, then the answer is fairly simple. Current (electrons) flow to the higher potential so initially, electrons (negative charge are attracted to a positive charge) would flow from the 12V battery to the 13V battery. This results in positive current flowing from the 13V battery to the 12V battery resulting in a depletion of charge from the 13V battery until both batteries are at 12V.

Yep. Kinda puts a dent in " highest voltage will dominate".

-Don- Reno, NV

 

[Great Horned Owl]

AH is how long the charge will last at a current rate. The total amp capacity can be different than the AH when time is NOT a factor.

"Cranking amps" is not cranking AH!

-Don- Reno, NV

NONSENSE

Battery capacity tell us how much charge (Coulombs) can be drawn from the battery. Charge simply can't be measured in Amperes. The Ampere is defined as 1 Coulomb per second. Asking how many Coulombs per second are stored in a battery in completely meaningless.

If you use an analogy that you will understand, the capacity of a water tank can be measured in gallons. Asking how many gallons per second are in a tank is pure nonsense.

You are right about one thing though. "'Cranking amps' is not cranking AH!" While correct, that statement is about as useful as saying "Miles per hour is not miles." A battery's cranking Amp rating has no relationship to its capacity. Rather, it states how much current the battery can put out in a short burst. It is far more dependent on the internal resistance of the battery, than on the capacity of the battery.

I do know what I'm talking about. I spent almost 50 years working as an electrical engineer. It's clear from your meaningless statements, that if you were ever taught the fundamentals of electricity, you have long since forgotten.

I don't care if you don't want to learn, but please refrain from giving out erroneous information that others will read and perhaps, believe. There's already too much confusion about the subjact

 

[DonTom]

Charge simply can't be measured in Amperes.

I think we are not communicating very well. Where did I say charge could be measured in amps?

Here, it talks about the difference between amp rating and AH of a battery. Are they wrong also, IYO?

Or was it that I should have used the word rating instead of capacity? "Capacity" can have more than one meaning.


-Don- Reno, NV

 

[Great Horned Owl]

No, you never said that "charge could be measured in amps." However, what you did say is equivalent. You said that battery capacity (which is charge) is measured in Amps. That's the same as saying that charge can be measured in Amps. Bty, you still haven't learned that Ampere is a proper noun.

The web page that you cite, is completely correct. However, you don't seem to understand it. That article actually contradicts what you have been arguing.

With respect to a battery, capacity CAN NOT have more than one meaning. The capacity of a battery refers to the amount charge that it can hold. There are, however many ways to express the amount of charge.

The standard unit of charge is the Coulomb, but most people aren't familiar with Coulombs, and we don't have a convenient way to measure them. By definition, 1 Ampere is equal to a flow of 1 Coulomb per second. Alternately, 1 Coulomb is the amount of charge that would be delivered by a 1 Amp current flowing for one second.

We could rate the capacity of a battery in terms of Ampere-seconds, but that would give a capacity of 720,000 Ampere-second capacity for what is more convenient to call a 200 Amp-hour battery. What we can't do is rate the capacity in Amps.

The "cranking power" rating of a battery is a poor choice of words, but it is what it is. A better term would probably be "peak current" rating. The instantaneous peak current has absolutely no relationship to the capacity of the battery. Rather, it is equal to the battery voltage divided by the battery internal resistance.


Actually, it's a little more complicated than that because the cranking power is how much current current can the battery deliver for 5 seconds, while maintaining a battery voltage of 7.5 volts. So, while the battery capacity is a factor, the two things are clearly not the same thing.

The word "rating," by itself has absolutely no meaning without specifying the type of rating. A battery has a capacity rating, a peak current rating, a voltage rating, a weight rating, and several others.

 

[OldWEB]

Ampere or amp is still ampere

Someone needs to go back to train school....

From The International System of Units...

5.2 Unit symbols
Unit symbols are printed in upright type regardless of the type used in the surrounding text.
They are printed in lower-case letters unless they are derived from a proper name, in which
case the first letter is a capital letter.
~~~~~~~~~~~~~~~~~~
5.3 Unit names
Unit names are normally printed in upright type and they are treated like ordinary nouns. In
English, the names of units start with a lower-case letter (even when the symbol for the unit
begins with a capital letter), except at the beginning of a sentence or in capitalized material
such as a title. In keeping with this rule, the correct spelling of the name of the unit with the
symbol °C is “degree Celsius” (the unit degree begins with a lower-case d and the modifier
Celsius begins with an upper-case C because it is a proper name).

 

[Contributor]

e, ie t

you still haven't learned hoto spell

hoto spell?

 

[DonTom]

Bty, you still haven't learned hoto spell

So it was only the word "capacity" you say I misused, which can have several meanings. I never meant the charge capacity in AH when I used the word "amps". Cranking amps is still a capacity of the battery and it is not an AH rating.

Did you notice that article, in the text, also did not always capitalize "amps"? I rarely see it capitalized and the same goes for watt, ohm, etc. which are also people's names. But here, I am not referring to people names so lower case is correct. And my dictionaries agree.

-Don- Reno, NV

 

[Great Horned Owl]

Ampere or amp is still ampere

Someone needs to go back to train school....

From The International System of Units...

5.2 Unit symbols
Unit symbols are printed in upright type regardless of the type used in the surrounding text.
They are printed in lower-case letters unless they are derived from a proper name, in which
case the first letter is a capital letter.
~~~~~~~~~~~~~~~~~~
5.3 Unit names
Unit names are normally printed in upright type and they are treated like ordinary nouns. In
English, the names of units start with a lower-case letter (even when the symbol for the unit
begins with a capital letter), except at the beginning of a sentence or in capitalized material
such as a title. In keeping with this rule, the correct spelling of the name of the unit with the
symbol °C is “degree Celsius” (the unit degree begins with a lower-case d and the modifier
Celsius begins with an upper-case C because it is a proper name)

I'll concede the point.

 

[Great Horned Owl]

So it was only the word "capacity" you say I misused, which can have several meanings. I never meant the charge capacity in AH when I used the word "amps". Cranking amps is still a capacity of the battery and it is not an AH rating.

Cranking amps is different from the capacity of a battery, It's a complicated rating which is a function of both the battery capacity and the battery internal resistance. It is a useful rating for the engine battery, but it is totally useless when talking about the house batteries, which don't even have such a rating.

It is possible to talk about either the current capacity of a battery, or the power (current times voltage) of a battery. However, the power capacity isn't often used.

From the Trojan Battery Company:
What are the 20-hour and 100-hour rates?
The 100-hour rate is just an index that is used in the battery industry to compare batteries of different types and sizes. The 100-hour rate is the amount of Ahs the battery will deliver during a 100-hour discharge. The capacity of a battery, in Ahs, is a dynamic number that is dependent on the discharge current. For example, a battery that is discharged at 10A will give you more capacity than a battery that is discharged at 100A. With the 100-hr rate, the battery is able to deliver more Ahs than with the 20-hr rate because the 100-hr rate uses a much lower discharge current than the 20-hr rate. Both rates are used as baselines in different parts of the world. Either rate, however, will give you the same view of a battery. A higher capacity battery will have higher 5 and 20 hour rates than a battery with lower capacity.

From Wikipedia:
A battery's capacity is the amount of electric charge it can deliver at the rated voltage. The more electrode material contained in the cell the greater its capacity. A small cell has less capacity than a larger cell with the same chemistry, although they develop the same open-circuit voltage.[50] Capacity is measured in units such as amp-hour (A·h). The rated capacity of a battery is usually expressed as the product of 20 hours multiplied by the current that a new battery can consistently supply for 20 hours at 68 °F (20 °C), while remaining above a specified terminal voltage per cell. For example, a battery rated at 100 A·h can deliver 5 A over a 20-hour period at room temperature. The fraction of the stored charge that a battery can deliver depends on multiple factors, including battery chemistry, the rate at which the charge is delivered (current), the required terminal voltage, the storage period, ambient temperature and other factors.

 

[DonTom]

Cranking amps is different from the capacity of a battery

I well understand all the ratings used with a battery, such as AH, RC, cranking amps, and everything else in your post. And even how an AH is NOT always an AH with a battery. They are usually spec'ed at the current draw that gives the best AH rating. Change the current draw a lot and the true AH capacity will normally lower.

However, the power capacity isn't often used.

If you mean KWH, it is used often with EVs. But that can have more than one meaning. The max capacity and the nominal capacity. The difference between the two cannot normally be used.

My 2018 Tesla M3 LR has a 70 KWH battery and that is the only rating /capacity I have ever heard mentioned with it. In this case, I don't even know if that is the max or nominal capacity as I really don't care as it has more than enough range for me.

-Don- Reno, NV

 

[Gary RV_Wizard]

You have two batteries. One measures 13 volts unloaded; the other measures 12 volts unloaded.

The battery voltage(s) aren't relevant to this question because the charging systems aren't operating at battery voltage. They are applying voltage & amps per their own output specs. And the OP is asking about the confluence of the two chargings systems at the house battery bank, so there is only one battery type involved here anyway. While traveling, the second charger source is the engine alternator voltage, which will typically be in the 13.4v-14.4v range. Influenced by the presence of the engine start battery, but not limited by it. While on shore power, it's the house converter/charger generating a second charge voltage. Each of those charge sources will see the combined system voltage as measured at the battery bank and will react according to the charging algorithm.