Two batteries wired in parallel

[bruceb58]

Re: Two batteries wired in parallel

the acr serves the same purpose as the isolator except that it does not isolate the batteries with the ign on and two unmatched batteries should not be charged in parallel

Both of these statements are just wrong.

First of all, the ACR only connects once the voltage of either battery gets up over a set threshold so it will not turn on with only the ignition.

Second, each battery will get the charge it needs based on how low the battery is. It will not matter what the size or capacity of each battery is as long as each battery is the same construction type(both agm or both lead/acid).

 

[smokeonthewater]

Re: Two batteries wired in parallel

Both of these statements are just wrong.

First of all, the ACR only connects once the voltage of either battery gets up over a set threshold so it will not turn on with only the ignition.

Second, each battery will get the charge it needs based on how low the battery is. It will not matter what the size or capacity of each battery is as long as each battery is the same construction type(both agm or both lead/acid).

Well bruce I did a fair bit of reading before putting this response together..... The first statement should read that the acr does not isolate the batteries while charging both.... That is not a particularly bad thing but rather just a difference.....

As for the second statement it is definately not incorrect.... the acr MAY correct for any problems with charging in parallel but I don't know for sure....

the references below are just the first couple I found. There are many more and likely better sources to be found.


Batteries of the same type only (engine starting, deep cycle, sealed, gelled, AGM, maintenance-free) can be charged at the same time in parallel (+ to +, - to -).
source

When connecting batteries in parallel be sure to use the same type of battery, for example; two deep cycle batteries. The age of the batteries should be the same. A new battery connected in parallel to an older one will not get fully charged, and the older battery may get overcharged.
source

 

[Silvertip]

Re: Two batteries wired in parallel

I totally disagree with the statement that two dissimilar batteries (age or capacity) cannot/should not be charged while connected in parallel. If this were true, then it would not be practical to run a marine starting battery and a deep cycle house battery on an ACR, combiner or switch which is standard practice. The source you posted showed a picture of how "new" batteries are gang-charged. Granted, one must assume they are all the same type in that example but the fact remains, each of the six cells in each of the large group of batteries cannot be made identical and therefore would take charging at a different rate, however slight than the others. Likewise an older battery connected in parallel to a newer battery and then charged with a single charger will indeed be charged fully and neither of them will over charge or undercharge (unless you stopped the charging process prematurely). The reason is quite simple. The charger cannot tell whether it is connected to the stronger or weaker battery (as relates to the state of charge). It therefore monitors system voltage which is a combination of the two batteries -- not each individual battery. In practice the weaker of the two will draw the largest amount of charge current and that will gradually taper off until the voltage of the two batteries begins to equalize (which may be somewhat less than 12.6 volts). From that point on they will will charge until both are at the fully charged level. The only time a parallel system may not charge correctly if one the two batteries is indeed failing (as in one or more bad cells). An older battery is not bad just because it is old. To help understand this concept, connecting two batteries in parallel and then walking away will result in the system voltage equalizing at the level of the lesser of the two. This equalization concept works the opposite way while charging except that the charger knows when the system level has been achieved. And just for the record, any flooded battery can be safely charged at a rate up to 20% of its AH capacity.

 

[smokeonthewater]

Re: Two batteries wired in parallel

I am getting tired of this debate so you have a real chance of "winning" just by wearing me down. You are wrong on several points and this question has been very well answered..... If you will take the time to research this you can easily find out where you are wrong.... batteries do not "fill" with charge in the manner that a glass fills with water.... as the battery nears fully charged gradually more and more amperage is converted to heat instead of being stored in the battery ... When batteries that charge at different rates are charged in paralel one battery CAN mask the higher terminal voltage of the other and induce an overcharge condition.... Also all batteries gradually deteriorate with every charge/discharge cycle and the charging properties of an older battery are not the same as a new one NONE of this is hard to find if you are willing to actually research..... There is ALOT more to this but I just don't care to type it.... also of course eagle eyes are scanning my every word just looking for a mistake to point out.....

There are LOTS of things that are "industry standard" but are WAY less than ideal.... such as cheap plastic bilge pump hoses and thru hull fitings... self draining decks with leaky hatches very close to the water line and poor fit and finish anywhere that is not readily noticed in a showroom

I have never stated that you could not use a batt switch only.... heck a pair of jumper cables will work just fine.... The system would likely lose a little of the service liffe of the batteries and would definately be less user friendly than an isolator and a switch but it would be cheaper and easier to install.....
I listed four options above starting with the one that I felt best and pointed out that you felt the last was best....

Please don't take offense. Every word I have typed here is only for the purpose of helping complete strangers and not at all to try to make you look bad.

 

[smokeonthewater]

Re: Two batteries wired in parallel

BTW in the diode iso vs the acr there are advantages of both and my choice for the diode iso is based on reliabilaty with no moving parts to fail

" Diode Versus Solenoid Battery Isolators
There are two main types of battery isolators: the diode isolator and the solenoid isolator. A diode isolator uses two high-current diodes to direct the flow of current from the battery to the battery isolator. A diode isolator is not as complex as a solenoid isolator, is easier to maintain and repair and will generally last longer. A solenoid isolator uses several electrical relays to control the flow of electrical current. One of the relays receives the electrical current from the battery while the other relay (known as the central relay) monitors the power levels of the idle batteries. Additional relays transmit the charging current from the alternator to the idle batteries. A solenoid battery isolator does not have as much current degradation as a diode isolator and is therefore more efficient. However, it is more complex and requires more maintenance than a diode battery isolator."
source

 

[smokeonthewater]

Re: Two batteries wired in parallel

The following is an excerpt from this article....... Read the whole article as it has alot of good points in it that directly apply to the original question.

"It is, therefore, the foolish one who sees it as impractical having dual anything aboard a vessel, until left in a predicament. So while two is generally accepted as better than one, it can well be said that one may be (removed the word "no" as I think it was an error) better than two, if the latter is improperly utilized.

For example, let's say you've been on an overnight fishing trip. Floodlights, bilge pump, aerator, spotlights, etc. have all been in active use the whole night. You have no electric generator for you are cleverly equipped with dual 12 volt batteries and switch.
Unfortunately, upon trying to crank the engine, you discover both batteries are dead due to the battery switch having been left in the "all" position, draining both batteries dead. This is the dilemma many find themselves in when managing their batteries with a battery switch alone. Also, if the battery switch was left in the "1" position (main cranking battery) while the "#2" (aux. battery) was drained from use, how will it be recharged by the alternator if the battery switch is not moved to either the "2" or "all" position?

Another problem with utilizing a battery switch alone for dual battery systems is that when the boat is not in use for an extended period of time and the battery switch is left in the "all" position, the weaker of the two batteries can drain the the stronger one. An even more serious problem, which can ruin the alternator, is placing the battery switch to the "off" position while the boat's engine is running."

 

[smokeonthewater]

Re: Two batteries wired in parallel

Here is another interesting article. It is obviously aimed at selling their product and is aimed at the rv market but has some good info in it.

"Q: What are the four types of battery isolators?
A: The first is simply a switch to remove the auxiliary battery from the car's electrical circuit. The disadvantage of this is that humans (such as me) forget to turn the switch on and off as appropriate.

The second is a diode isolator. This is simple, it allows current to flow from the circuit with the highest voltage. The disadvantages of these are limited current, and the fact that there is always a half-volt drop across the resistors. edit, this is outdated, modern diode iso's have a voltage drop of less than .05 volts and thus thus throws off the numbers in the rest of this paragraph by 90% This will dissipate 50 watts when 100 amps are flowing, so it is wasteful unless you are using that power to heat the trailer. It also lowers the charge voltage going to the auxiliary battery which severely decreases its maximum charge rate.

The third is a solid-state relay system that uses control circuitry and power MOSFETS to do the switching. This eliminates the diode drop, though there is still some resistance in the power transistor conduction channels to cause heat and to limit the amount of current to flow. These are susceptible to damage from voltage and current spikes and can be damaged by drawing too much current through them. Due to the high cost of high-current MOSFETs these isolators are about twice the price and half the pass-through current of the hybrid unit that we sell.

The fourth is our hybrid system that uses a microprocessor circuit to monitor the charging and discharging and a rugged, reliable solenoid contactor (relay) to allow enormous currents to flow without damaging the electronics. This has the advantage of high precision, high reliability and low cost. "

 

[smokeonthewater]

Re: Two batteries wired in parallel

Here is the source for the following diagram. There are several more complex diagrams along with applications for them on that page

BATTERY SELECTOR SWITCH AND MASTER DISCONNECT WITH AUTOMATIC BATTERY ISOLATOR AND AUTOMATIC CHARGING - TWO BATTERIES - #D
Installation of an isolator automatically isolates each battery, lower charged battery will automatically be charged first. Two different size batteries can be used, larger for ship services, smaller for engine starting. Switch selects usage, master disconnect, "power" parallel function.

 

[bruceb58]

Re: Two batteries wired in parallel

I am getting tired of this debate so you have a real chance of "winning" just by wearing me down.

Now that is almost funny considering all the posts you just submitted.

A lot of the sources you quoted have just bad info. Not sure where you get that fact that a diode has a forward voltage of 0.05V. That is just plain wrong.

Charging a starting battery in parallel with a deep discharge battery is not a problem. They both have the exact same fully charged voltage!

I am an electrical engineer. Almost everyone of these sources has incorrect info in them. Not surprising. The last Powerstream source was really bad. Definitely not written my anyone with much electronics knowledge.

Bottomline...the OP should by the BlueSeas switch and the ACR. The diode battery isolator is that last method that should be used. It will not let your batteries get fully charged.

 

[smokeonthewater]

Re: Two batteries wired in parallel

Now that is almost funny considerring all the posts you just submitted.

A lot of the sources you quoted have just bad info. Not sure where you get that fact that a diode has a forward voltage of 0.05V. That is just plain wrong.

Charging a starting battery in parallel with a deep discharge battery is not a problem. They both have the exact same fully charged voltage!

well dangit i certainly don't mean to be funny and I had decided to just drop this at about post 5 but I looked at the number of veiws and that makes ALOT of misinformed people so I thought I would try again to explain. on the other hand this just keeps going.... The term btw is forward voltage DROP. I expect that you knew that but many folks reading may not... specifically I did not say diodes have a fvd of .05, I said the the newest diode block "isolators" have a voltage drop of less than .05... I got that info from the websites of the manufacturers of the battery isolators I was reading about... I did post one of those as a source.... I suppose they could be lying......There were some that suggested the use of a different voltage regulator to compensate for the voltage drop on their older tech isolators....

I guess I could go actually test the voltage drop on my own boat with a dvom although since I bought a cheapo from auto zone there's no telling at this point whether it would help my case or not

Honestly though we are actually in agreement on my MAIN point which is battery switch AND isolator (be it relays or diodes)

I bought my carver as a project with mose of the engine and 110/12v house wiring hacked out and likely scrapped by some junkie. I rebuilt it wire by wire from scratch without any diagrams or help of any kind..... While there may be room for improvement somewhere it works very well as is. On the new boat I will be running another diode isolator and if I can not actually purchase one with .05 fvd then I will use a voltage regulator that properly sets voltage to reach the batteries at optimal charge voltage
but I certainly would not think badly about someone who chose an acr. These devices both have good points and bad and which of those is more important will be a personal decision the guy who owns the boat and designs the system.

I have known and worked with ALOT of engineers from many disciplines and the best ones were the rare few that believed there were things they didn't know and thus were not above discussing with a layman as an equal. With that attitude much can be learned on both sides..... That said I am always open to learn new things myself. Show me some reference on my errors and I will actually enjoy reading it....
Just pointing and saying "that isn't true" doesn't teach anyone anything. Admittedly I have taken manufacturer and published data at face value but I haven't found anything to refute it and I have looked ALOT..... by all means SHOW ME.... I have no hidden agenda but rather am only interested in putting together the best information available.....

 

[bruceb58]

Re: Two batteries wired in parallel

The term btw is forward voltage DROP... I got that info from the websites of the manufacturers of the battery isolators I was reading about

Before you try educating me with what the correct term is, maybe you should read an actual diode datasheet. Go to page 2 of the this data sheet.

http://www.experimentalistsanonymous.com/diy/Datasheets/1N4148.pdf

 

[bruceb58]

Re: Two batteries wired in parallel

Therre were some that suggested the use of a different voltage regulator to compensate for the voltage drop on their older tech isolators....

Yes....That is what they USED to do with the diode isolators...20 years ago!. That was the OLD days when they didn't have high current ACRs available that are inexpensive. That is why you don't want to use them.

 

[smokeonthewater]

Re: Two batteries wired in parallel

I did a big edit while you were posting those last two.... well I have seen it listed both ways but have always seen it as voltage drop or forward voltage drop in print and vf on tables or spec lists.... I didn't intend to educate you but rather to simplify for those who have more dificulty following and I had tried to clarify that with the edit

 

[smokeonthewater]

Re: Two batteries wired in parallel

Yes....That is what they USED to do with the diode isolators...20 years ago!. That was the OLD days when they didn't have high current ACRs available that are inexpensive. That is why you don't want to use them.

read post 17

 

[northernmerc]

Re: Two batteries wired in parallel

This is an interesting but confusing discussion. Maybe when I'm less tired, I'll read through it in more detail.

Or I may just keep on doing what I already do - carry an extra battery and booster cables, or just switch batteries if the need arises.

 

[smokeonthewater]

Re: Two batteries wired in parallel

I am sorry this has become so confusing


Bruce I had a feeling I was forgetting something so I just went out and crawled into the engine compartment of my carver.... My isolator is the older style simple diode block but it has a terminal for the voltage sensing wire of the voltage regulator..... hooking that up causes the regulator to sense the voltage properly from the output side of the iso and thus charge the batts properly and fully....only downsides left are a little wasted energy in the form of heat and one more wire connection to make. The advantage tho is still higher relliabilaty with no moving parts or contactors to burn or corrode

 

[bruceb58]

Re: Two batteries wired in parallel

I am sorry this has become so confusing


Bruce I had a feeling I was forgetting something so I just went out and crawled into the engine compartment of my carver.... My isolator is the older style simple diode block but it has a terminal for the voltage sensing wire of the voltage regulator..... hooking that up causes the regulator to sense the voltage properly from the output side of the iso and thus charge the batts properly and fully....only downsides left are a little wasted energy in the form of heat and one more wire connection to make. The advantage tho is still higher relliabilaty with no moving parts or contactors to burn or corrode

Its only going to sense one battery. So...you have to decide which battery you think is going to be your lowest one most of the time. Its a compromise that the diode isolator people tried to get around the voltage drop issue. If you happen to have a one wire alternator on your boat with no external sense you also can't use this.

The problem with the unit from the English source is that if you have a component failure, your alternator can be running without a load.

An ACR is so simple. I have been using them for years. RVs have been using basically the same principal. What they do is have the electronics that sense the alternator voltage rising over 13V to turn on a relay that charges the house batteries. RVs gave up on diode isolators years ago. The only reason you don't see them in boats is because they aren't sealed units so you have the possibility of an explosion from igniting gas fumes.

BTW in the diode iso vs the acr there are advantages of both and my choice for the diode iso is based on reliabilaty with no moving parts to fail

An ACR is also a solid state device with no moving parts to fail.

 

[smokeonthewater]

Re: Two batteries wired in parallel

well either way I'm sure we can agree that either choice is a tradeoff and a matter of application specific personal preference.....My iso does not require the choosing of which bank to sense from but rather it has an additional lug for the sensor wire..... Now whether that it directly connected to one of the outputs internally I don't yet know. I do know that I get the same voltage on both the start battery and the house bank. If you had to though choosing the lower charged battery bank is easy.... it's the house battery. If you somehow manage to run down your starting battery then you still have the both setting on the batt switch for emergency..... if you aren't regularly using more of the house bank than the starting batt then you don't need a house bank but rather a backup starting battery which would be wired differently
the acr could have a component failure leaving the alternator with no load more readily than such a failure on the example mosfet assisted diode block..... even if the mosfets were to burn out I suspect although I can't say for sure that there would still be a partial load through the diodes

 

[bruceb58]

Re: Two batteries wired in parallel

the acr could have a component failure leaving the alternator with no load more readily than such a failure on the example mosfet assisted diode blockdes

Impossible since the ACR is just wired between the two positive posts of the batteries. If the ACR goes out, you just don't have a connection anymore between the two batteries when the alternator is charging.

There is no such thing as a mosfet assisted diode block. That is some sort of name someone made up to sell people that don't know any better thinking they are getting a diode isolator on steroids.

As far as a preference...I agree...you can have the old diode isolator that doesn't work as well as an ACR or you can use the ACR.

 

[smokeonthewater]

Re: Two batteries wired in parallel

Impossible since the ACR is just wired between the two positive posts of the batteries. If the ACR goes out, you just don't have a connection anymore between the two batteries when the alternator is charging.

There is no such thing as a mosfet assisted diode block. That is some sort of name someone made up to sell people that don't know any better thinking they are getting a diode isolator on steroids.
actually it was a term I used to try to somehow shorten this long debate a little

As far as a preference...I agree...you can have the old diode isolator that doesn't work as well as an ACR or you can use the ACR.

Of course you do..its also 0.7V lower than what your alternator is putting out and therefore also not fully charged.
I see you went back and edited this out..... I guess you can make mistakes too huh

many acr's do connect between the alt and the batts so it is NOT impossible for them to fail to link them There are simpler units that simply jump to the second battery so no that style could not interupt the connection to the first batt Of course I'm sure that is the style you would choose but a blanket statement that an acr can't fail and cause alternator failure is just not true

A properly installed diode iso will have provisions for proper charge of the batteries either by alt sensing on the downstream side of the iso or as in the european example with mosfets