How does the Multiplus trickle charger work

DIY
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The trickle charger on the Mutliplus is not a true charger, it does not have a set charge voltage. It is a small circuit that is connected directly to the DC input header within the Multiplus and it contains a current limiter and a diode. The diode is to prevent the starter battery from charging the domestic battery if the domestic battery has the lower voltage. See the diagram below. This means that the trickle charge function is always active, therefore if you have solar connected to your domestic batteries then when the solar charger increases the domestic battery voltage, then the trickle charger passes some of that charge to the starter battery.

NOTE: for the trickle charger to work, the starter battery and the domestic battery need a common negative to complete the circuit. If you have a shunt fitted on the domestic batteries then the starter battery MUST be connected to the “System” / “Load” side of the shunt.

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The diode in the trickle charger has a voltage drop of around 0.3V before any trickle charge current will flow, increasing to around 0.6V when the trickle charge current is higher. When both the domestic and starter batteries are lead acid this means that the trickle charge function really only works when the domestic battery has some charge taking place lifting the domestic battery above 13.1V when the starter battery is full at 12.8V (for a 12V system).

The trickle charge function is not recommended if the domestic battery is lithium and the starter battery is lead acid due to the higher voltage of lithium batteries. With a lithium battery having a working voltage range of 13.1-13.5V (for a 12V system) means that the trickle charge will permanently pass a current to the starter battery even if the starter battery is full. Additionally, the trickle charger is not controlled by the Mutliplus and will not shut down if DVCC tells the Mutliplus to stop discharging the domestic battery. With some BMSs such as the VE Bus BMS (V1 or V2) and Smart BMS CL 12/100 which do not isolate loads but control the Multiplus via DVCC or ATC/ATD input wires, this means that the trickle charge would continue to drain the domestic battery after a load disconnect had occurred.

IF your system has the capability to provide the energy for a permanent 13.2V storage charge on the starter battery AND has positive disconnection of the DC supply to the Multiplus (Lynx BMS or batteries with built in BMS disconnection), then the trickle charger may be used with a lithium domestic battery and lead acid starter battery.

The trickle charger must not be used if the starter battery is lithium because it has no control over the charging voltage or current.

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The graphs above illustrates how the trickle charger works showing 1 day starting at midnight. The domestic battery is being discharged and the voltage has fallen to 12.7V, below the starter battery at 12.9V which remains full after the previous days engine run. At this stage there is no trickle charging and the starter battery voltage is slowly falling. At 07:00 solar charging of the domestic battery starts and the domestic battery voltage increases. When the domestic battery voltage reaches 13.2V the starter battery voltage starts to increase (0.3V drop due to the diode). After this point the starter battery voltage increases and follows the domestic battery voltage. When the domestic battery voltage reaches 14.4V, the starter battery voltage is at 13.8V, at this point there is a higher trickle charge current and the diode is dropping 0.6V. Between 12:00 and 17:00hrs the air conditioning is turned on using all the solar power along with some load on the domestic batteries, trickle charging stops and the starter battery voltage decays. The air conditioning is turned off at 17:00 and solar charging returns and the starter battery also receives some charge with solar charging ending at 19:00.

NOTE,
the description and illustration above are all based on a 12V Multiplus with a 12V starter battery. As the trickle charger is a direct take off from the input DC bus, a 24V Multiplus will only be able to charge a 24V starter battery.

12 Likes
6

Best write-up I’ve seen that explains how the trickle charger (aka Aux port) works.

1 Like
7

Hello there!
May I ask you explain that part in details or should I say more in a practical way? How would I achieve that? What is the positive disconnection?

I would like to make this trickle charger work with lead acid starter and LiFePO4 house battery. Our boat will be on the water unattended for weeks but plugged into the shore power outlet and we need something to keep up the starter battery.
I was looking at the Orion solution, but I woul love to solve that with some tricks with this trickle charge option :smiley:

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On a lithium battery you need some protection against discharging them to a low voltage and damaging them. Many batteries have an internal BMS which shuts off the battery discharge on low voltage to protect the battery. If you have this you are fine. If you are using Victron batteries with an external BMS then only the Lynx BMS provides adequate shutdown. If you do not understand this basic issue with lithiums then you need to research some more.

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Ok, now I get you. Thank you for the information. Is there a better solution to charge starter battery? How would you do it for the boat which is docked far away and it has solar and shore power which are charging LiFePO4 battery?

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Your description also applies to my boat, lithium to lead. I am in the UK and fitted a device from Ablemail.not sure if it is UK only.

https://ablemail.co.uk/products/amt12-2-trickle-charger-charges-vehicle-battery-from-leisure-battery-with-solar-charger-mains-hookup/

11

Ismot the same with aux output on the quattro? Same system?

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I do not know about the Quattro.

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I don’t believe the 2.charger output is labelled as “aux” .

14

I use an LFP house bank and AGM starter, and the aux charge has gone up to 17v before…
I definitely need to make my setup smarter.

15

Hello,

@pwfarnell thanks a lot for your detailed explanation, it really helps a lot. I have two questions based on that.

So in your opinion the let’s say “overcharging” of the AGM batterie by the Lithium Battery via the trickle charge of MP in your example, isn’t a problem for the AGM or? Could you explain why?

And my more important question, if I would control a smart battery protect or cyrix with Load Disconnect port of the Victron VE.Bus BMS-V2, and I would run the trickle charge cable over it. This would prevent the lithium battery to be totally discharged or? Because if the BMS says “load disconnect” then there will no energy be flow to the starter battery or?

16

I do not believe that an AGM will be overcharged because the rested full voltage of a lithium is 13.6V, so an AGM starter battery will be kept at 13.3V at most, which is a storage voltage, lower than the typical 13.5-13.6V float.

Your idea of passing the charge current through a switching device controlled by the BMS ATD would solve the discharge problem. I suggest you look at the Victron SolidSwitch, it can be activated by a low switching current and is smaller and cheaper than the battery protect or Cyrix, it is more in line with the small trickle charger current.

17

Does the trickle charger on the IP43 Phoenix Smart Charger work similarly?

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if this pic comes out.. this is a 150/85 victron MPPT set for 14.2v/13.4v with a tail current of 1 amp, and you can see the Starter battery stepdown/AGM voltage in a sprinter MH.. There is approx 15 ft of #14ga wire and a switch on it.. it runs 13.4v down to 12.85 ish in its daily cycles.. When you go to start the vehicle, turn off the trickle charge with your switch(highly recommended), and let the bat rest a wee bit, then the “smart modules” wont have issue when it begins to alterntor discharg.. and because of the shunt, no triggering or reverse flow happens.. and finally, the DC to DC for the chassis battery to alternator charge off the main alternator is wire triggered vs voltage triggered, so no voltage loop can occur.. it works perfectly.. I keep the Cerbo, a small modem, and let the MP sit in stand by thusly pulling around 15 to 20 watts when the coach is parked in outdoor storage.. the MPPT initiates a bulk charge every morning, so I wanted to minimize the absorption time while it sits, and the 8 cells rarely get out of any balance. I leave the Inverter set at 14.4v and abs of 2 hrs, for when I plug it in, and the starter will go to 13.45 for that time, or I just switch it off when I am in the coach.. fwiw.. it works..

19

Hi there,
Can the Multiplus-II 24V trickle charger line be used to trickle charge a 12V starter/engine battery? I would assume that it can’t and realize this thread is old but can’t find info on the Multiplus-ii 24/3000 70-50 2x120v trickle charge feature.
Thanks!

1 Like
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@bud As the trickle charger is a direct take off from the input DC bus, a 24V Multiplus will only be able to charge a 24V starter battery. This is a good question and this has prompted me to add this note to the original post.

21

Thx. From your post it appears it is always enabled, correct? I think I will test it with a voltmeter.

22

Yes, it is always on.

23

As this is a constant voltage with a fairly high impedance, you might connect an mppt 75/1* to this and charge the starterbattery from there.

Always worth a try.