Temporally Isolating the Dual Battery Isolator. For 24V sys

[.:Dave:.]

My Problem

I need to run a 24V Hydraulic Power Pack. It will not be used very often and only for short periods of time. While the Hydraulic pump is not being used. I want the dual battery system to function as per normal.


Proposed Solution

Have a normal dual battery system. Have an NC relay on the dual isolator cable and Aux Battery Neg cable and a NO on a series connection between the two batteries. When I need 24V, switch the three relays (protecting the dual isolator) and get 24V from a series connection of the two batteries.



Will this blow anything up? Will I have issues using 12V (stereo etc) from 1 battery while I get 24V from the two?


I understand the purpose of the dual battery system in that it is meant to keep your starter battery charged. I can see problems associated with one battery being more charged then the other and the current flows between the batteries in this case.

I want to get 24V without running a three battery system.

This is a hydraulic suspension related project and I only will need 24V whilst the car is running. (Wont be draining battery when the car is off).

I have read most of the 24V related posts in the forum.

Your opinion and guidance towards this setup will be greatly appreciated.

Dave
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[ausoops]

that will work, you also need to isolate the two +12v legs from each other as they will be a dead short if they are both connected to the isolator you can use one relay to achieve this, just open the +24v posistive leg to the Dual battery isolator,
also dont be tempted to use a double pole (ie NO,NC) relay, i would use two mechanically and electrically interlocked relays for your 24/12 switching also ensure that they are break before make relays, the last thing you want is a short as the batteries will feed fault current until it melts the cable off.

any 12v load connected to the aux battery will need to be disconnected when in 24v mode as they will become a dead short across the aux battery however these can be connected between the new relay on the +24v postive leg and the Dbs ISOLATOR, this will transfer 12v load from the aux battery to the main battery when running 24v. ignition and load of the main battery will be fine. centre taping a 24v system for 12v is usually not a good idea due to bad discharge/charging rates, but given it will return to parallel charging it shouldn't root the batteries at all.

what sort of load is this pump going to put on the bank? and how long, the longer your in 24v mode the less volts you will have for ignition and the like.

[ausoops]

however what is being proposed is very expensive and will require a full rewire of your dual battery system and installation of new relays and control circuits. can you just change the pump motor to 12v?

[ausoops]

very rough i just whipped up

[drivesafe]

You are going to need a third battery for one simple reason, your present idea requires the the alternator to be disconnected with the motor running.

The instant you do this your alternator will be stuffed, your alternator MUST be connected to a 12v battery at all times while the motor is running.

[.:Dave:.]

Thanks for the replies.

I currently don’t have a dual Battery system installed at the moment, so everything will go in fresh. Part wise, my dad is an electrician (240V – Industrial) so I was hoping to get the relays and such through his company. Ill just buy the isolator from somewhere else.

I have not really looked at the getting a different motor for the pump. The 24V one was cheap off eBay (I now know why) and a new 12V one is about $1000. I might check if I can just get the motor part to bolt on.

This is a uni project so I was trying to do it on the cheap through combining what have/can get (although it might not be the most elegant solution). I bought the 24V one as an excuse to get a Dual Bat system but did not realise the difficulty in the setup.

I have to run hydraulic control solenoids and other circuitry whilst the pump is operating. Is this going to be an issue with the 12V link you proposed above?


I currently don’t have the pump with me so ill have to get back to you on the load. Time will easily be less than 30 Sec.

In regards to the drivesafe’s post, can I run the control circuity directly off the alternator thus pulling some load from it? Will this solve the disconnection issue? (Assuming the issue is that you always need a load on there)

All this effort for 50mm lift. :(

Thanks for your help

Dave
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[ausoops]

You are going to need a third battery for one simple reason, your present idea requires the the alternator to be disconnected with the motor running.

The instant you do this your alternator will be stuffed, your alternator MUST be connected to a 12v battery at all times while the motor is running.

why? i thought if you disconnect it completely ie no field excitation, it should be fine, just spin and generate no voltage

now that i think about it though a lot of alts are self exciting so you would need to interupt the excitor circuit -i think?

[-Scott-]

You are going to need a third battery for one simple reason, your present idea requires the the alternator to be disconnected with the motor running.

The instant you do this your alternator will be stuffed, your alternator MUST be connected to a 12v battery at all times while the motor is running.

The third battery wouldn't need to be big, would it? Some form of motorbike battery?

You are going to need a third battery for one simple reason, your present idea requires the the alternator to be disconnected with the motor running.

The instant you do this your alternator will be stuffed, your alternator MUST be connected to a 12v battery at all times while the motor is running.

why? i thought if you disconnect it completely ie no field excitation, it should be fine, just spin and generate no voltage

now that i think about it though a lot of alts are self exciting so you would need to interupt the excitor circuit -i think?

That's what I would expect. Would opening the field circuit for 24V operation protect the alternator? I realise this could be tricky with an internal regulator, but how difficult would it be to find/convert to an external regulator configuration?

I imagine all the relays would need to switch in a specific sequence: open field circuit, disconnect earth from one battery, close the series connection.

I think a small third battery could be the simpler solution.

[ausoops]

its a lot of rooting around to get one 24v load happening. just change the motor to 12v, or depending on load, get a switchmode power supply 12-24vdc, you can get units up to 30-40Amps.

[.:Dave:.]

Thanks for the help everyone,


Ill way up the options and get back to you. Will have to sort out Price vs. ease of installation.


Cheers

Dave
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[chimpboy]

The other problem, and it is imho a big one, is that your setup has bad failure modes. That is, if one of your relays fails to operate, you will end up with a battery and/or the alternator in a complete dead short.

Relays do fail, it has happened to all of us, and it would happen to you sooner or later. They can stick in either position, or a trigger wire to them can break, or whatever. Look at each of the relays there and ask, "what would happen if this relay failed to trip but the rest worked? what would happen if it tripped but then didn't de-activate while the rest of the system worked?"

The only way you can confidently set something like this up is to come up with a system where your planned wiring is safe (even if it stops working) no matter which position each of the relays is in.

I would be more comfortable with something like the below, using only one double pole, double throw, NC relay where the two latches are mechanically linked.



That is just one relay with two poles. With this setup, the green path is a 12V system while the blue path is a 24V system. It is a bit simpler to look at on paper but the main advantage is that it is much safer in the case of a relay failure, and there's no interfering with the alternator so you have normal 12V operation the whole time. All your 12V stuff MUST run off the main batt only, the aux one only drops back to 12V for charging. You would not need matched batteries or even a full-size aux battery.

I have just drawn this up out of my head though so look it over in case I've made a mistake.

Alternatively have you considered tapping into the hydraulic pump for the power steering?

Other options woudl be a 24V alternator, or a separate 24V battery pack that you charge with a small 12-24V adaptor.