Multi-Charging Batteries

If the batteries are completely separate, using three separate battery chargers.

Are these batteries different voltages or are they isolated batteries for an auxiliary power supply (eg- single 12V car battery for main vehicle, 2x12V series connected batteries for a 24V auxiliary power supply for a camping van or similar)? If so then you need an additional 12V to 24V battery charger in parallel with your existing 12V alternator / vehicle battery system to recharge your auxiliary 24V supply when the engine is running.

If the batteries are all the same voltage you can install an isolator and the batteries will charge separately and at the rates each one needs. They will charge slower than a single one because the alternator output will be shared between them all. If the voltages are different and any of them is a higher voltage than the alternator then you will have problems and need to install a second form of charging for the higher voltage. Another factor you need to consider is the total amperage needing to be charged and the running time of your engine/alternator to be sure that the job gets done. Say for instance you have three 800 amp hour batteries and your alternator is 70 amps output. You would need to run your engine for over 30 hours just to charge the batteries and that is not taking into account that some of the alternator output will be needed to run the engine and vehicle's needs! You might be creating a situation where you cannot generate enough output to do the job in the running times you will be driving! Here is my direct Email, if you could give me more specific information I would be happy to assist you. I have over 35 years experience as an automotive engineer. johnd7618@cox.net

John D,

It would be the cat's meow if you were to log on and share you expertise with us.

This is The Engineer's place for News and discussion.

Jon

John D

If the batteries are all the same voltage you can install an isolator and the batteries will charge separately and at the rates each one needs.

The isolator is used to keep the batteries separate when they are not charging. They all receive the same charging voltage while charging. If a cell in one battery is shorted all the batteries will be effected by the lower voltage caused by the increased load and resultant voltage drop in the charging supply. The isolator has little effect on that.

A hefty three pole relay that is energized during charging and connects all three batteries together and opens when the charging supply is off would provide the same effect.

The solid state isolator is used because it is simple and trouble free.

If the batteries are charged AND used connected in parallel an isolator has no purpose.

If they are charged in parallel and used in series that question was dealt with in an earlier post.

Jon

Laosis,

"How can an alternator charge 3 batteries separately and at different rates when required?"

This looks like another guessing game. What I mean is someone asks a question and leaves out significant details. Like the details of the Alternator, like output voltage and maximum current, details of each battery such as Voltage and Amp hour rating, lead acid or sealed lead acid.

Do you want to charge one at a time based on their Voltage and Amp hour rating?

Like one at 0.3Amps and one at 3 amps and one at 10Amps?

No problem if they are all the same Voltage. Charge them all from the same alternator of the properly rated output for your expected load. The batteries will typically respond at their own rates. You know, ~14 volt alternator for 12 Volt batteries is typical. If they are all drained they will demand a lot of current for a while and then reach a voltage that allows them to charge at a comfortable rate for a longer time until they are all satisfied. (Batteries with emotions? Hmmm...)

For 2-battery systems. Automatically charges auxiliary battery as you drive, never overcharges . Self-contained electrical system converts alternator or generator to safe, efficient charger . Automatically isolate both the auxiliary and vehicle batteries so the discharge of one does not drain the other's power. Handle alternators up to 120 amps. Rectifiers and heat sink allow maximum heat dissipation without mechanical attachment. Eliminate shorting problems. Solid state. For all 6- to 36-volt, negative-ground electrical systems. UL listed. Connector kit required,16114G (sold separately).

If you are charging at 12Volts connected in parallel and using them connected in series for 24 volts you can use an "Orpin" to transfer the connections. I have seen this on dump trucks for starting at 24 Volts whith a 24volt starter while everything else was connected to one battery to operate at 12Volts.

Try JC Whitney DUAL-VOLTAGE STARTING SWITCH

http://www.jcwhitney.com/autoparts/ProductBrowse?storeId=10101&Pr=p_Product.CATENTRY_ID:2006120&productId=2006120&catalogId=10101

Energize it for 24 volts, de-energize for 12Volts.

Dual-Voltage Starting Switch 12- to 24-volt

How about you Jack?

Jon

In addition to what you have been told in previous posts, consider how the alternator will handle the increased load. Most car alternators are designed to just handle the electrical load of that vehicle. There is little extra capacity. If you now start adding more and bigger batteries, you could end up burning out the alternator. I have seen this happen on boats when the owner adds a big house bank but uses the OEM alternator.

Elnav,

I should have bolded and underlined that part of my entry.

alternator of the properly rated output

There are some mighty hefty aftermarket alternators out there.

Jon

Elnav,

Good idea to include a fuselink?

Jon

Yes. It goes without saying over current protection must always be used. Only starter circuits are hard to protect. But even they could be protected if you know the maximum workign load current. Question becomes how much do you expend on over current protection?

Elnav,

Yes, it is not something you want to do after the smoke clears.

Fusing for the output rating of the alternator seems like a good idea.

The old regulators for generators had voltage and current limiting relays.

If the automotive engineer would log in to share we could ask about the modern charging systems and how they manage current limiting when the load is too great.

Jon

Kududweller wrote: we could ask about the modern charging systems and how they manage current limiting when the load is too great.

My own observation is that no such limiting exist in most cases. I have seen fusible links on some but not all modern vehicles. Generally speaking the system tends to be self limiting. As long as the system remains in the original configuration everything is fine. However when someone alters it that is when trouble begins.

Highway trucks run by Owner/Operators is a good example. Some of them are decorated with rows of lights that make for a spectacular display. None of the guys I spoke to had any clue how much current that consumed. A few said it must be okay since they still had 12V in the battery. Wrong. 12V is not sufficient to CHARGE a 12V battery. Many operators complain about having to replace batteries or alternators at one year or 18 month intervals. Guess why?

A study found that many of the fleet rigs suffered from chronic under charging of the battery. When the lights were on, the system voltage would be dragged down to 13.0 volts. Barely enough to create a charge for the next start cycle. During daytime the voltage would rise to maybe 13.5 V and that saved them.

In the marine world where people quite often load down the battery and alternator even more severrely with electrical stuff including powering AC loads with an inverter, total destruction of alternators is more common. I saw one alternator where the stator windings got so hot the wires melted and formed little balls of molten metal before the heat dropped and the balls solidified in that shape.

Fixed voltage regulators also tend to be more self limiting and thus save themselves.

As the load increases, the voltage folds back and this has the effect of reducing the current flow. Load down the alternator enough and the voltage collapses. Alternators are by definition considered self limiting and that is why you rarely see fusing on the output. However, modify the system - especially with inverter loads, and you upset the applecart. Inverters tend to pull more current as the supply voltage drops. As long as the battery can help this is not a big problem but when the battery is exhausted then the whole system starts to collapse. Meanwhile the poor alternator gets abused and eventually overheats and occasionally self destructs from excess heat.

My question to the original poster remains. What is he planning to drive with three seperate batteries? Has he considered the loading this places on the OEM Bosch alternator? Just because the unit is 'rated' at 85 Amps does not mean it can sustain 85 amps for long periods of time.

Thanks very much for the advice. Its most appreciated. Sorry about the bad info.

Voltage 12 volts on all three systems.

Alternator Bosch 85amp max output. Alternator will run at 4000 rpm constant.

Battries are two 70 ah and one 85 ah.

Current drawn will never exceed 50/55 amp max. Except when starting.

Laois,

What Amp hour rating are the 3 batteries?

Are they car batteries?

Jon

Laois you haven't explained what the battery banks power. Granted that 2 x 70 + 85 only totals 225 Amp hour but how much and how fast do you expect to extract that energy? The total capacity is about the same as one big truck battery. What is the reason for making it a more complex 3 bank system? The advice you get is only going to be as good as the data you provide. Incomplete data = incomplete solution.

Laois,

That was strange........ Asking what amp hour rating when you already gave it.

Jon

Laois,

Sooo...

You have an 85 amp alternator that will run at 4000 rpm constant.

The alternator is providing 50 to 55 amps to a load while running and some of the current is used to recharge the batteries after they were used for starting the alternator or keep them topped off.

You have 3 batteries connected in parallel that are connected to the alternator charging circuit.

The batteries are used to re-start the alternator.

Are the batteries used to provide power when the alternator off for servicing?

Jon

No one so far has raised the point of voltage drop. If you use a diode block separator you will lose 0.7 to 1V over the junctions this will leave your batteries always at a low state of charge. This is very noticeable on 12 V systems.

This is improvable by selecting low voltage drop isolators, typically 0.3V.

Better still use a MOSFET version which is usually marketed as "zero Volt drop". Of course they cannot be zero but they are a great improvement with a typical loss of 0.1V or less at full load. As they have a linear resistive characteristic the voltage drop reduces with the current. All of the above are available with three outputs.

Another possibility is to use a voltage sensitive relay. These are available with the capability to handle the output of your alternator but you will need two units. The voltage drop will be that of the contact resistance.

Try looking at:- www.mastervolt.com , www.Victronenergy.com and www.balmar.net

I noticed on the Balmar site they advise the use of separately exited alternator with the voltage sense at the batteries. On first view this seems a good idea but I have seen several occasions where either a failure of the sense connection or of an output of the isolator causes the regulator to stay full on even when two of the batteries are being over charged. The best way to tackle this is to sense the voltage at the input to the isolator and set the regulator higher to account for the voltage drop.

regards

Chas