Two DC-DC chargers in Parallel

Hylife · Guru

Two DC-DC chargers in Parallel

GerryP · Veteran Member

I haven't actually tried two DC-DC chargers together, but it would be similar to having say a DC (or even AC) charger together with a solar controller, both on the same battery. You will get some additional benefit, but you most likely will not double your charge rate.

What usually happens is that instead of seeing the battery voltage, they will see the other and one will probably decide that to 'back off' as it thinks the battery is at a higher voltage than it actually would be without the other charger connected.

It won't hurt anything and the battery will still charge, but I fear you won't get what you are hoping for.

Peter_n_Margaret · Guru

I would not anticipate any problem.

Cheers,
Peter

patrol03 · Senior Member

If you could give an indication of the type of vehicle you drive and the items you need to supply current to would be helpful in providing advice in your situation.

Hylife · Guru

patrol03 wrote:
If you could give an indication of the type of vehicle you drive and the items you need to supply current to would be helpful in providing advice in your situation.

I certainly appreciated advice from lots of very helpful well meaning folks who "think" it might work, but because I've already asked a couple of auto electricians who basically said "dunno, neva' tried it, would depend on the charger", I was after feedback from someone who has actually done it.

Unfortunately, smart circuit design of todays' DC-DC multi-stage chargers might mean parallel charging just doesn't function as expected or at all. Rather than me wasting my money re-inventing the wheel (and possibly failing), if there is someone out there who has done/tried running two units in parallel, thats who I'd like to hear from.

As lower powered devices are available in a bigger range of brands, are more competitively priced and are disproportionately much cheaper than higher powered ones, my thoughts were to purchase another identical 20amp DC-DC charger (approx $160) to the one I already have and to run two 20amp units in parallel, rather than fork out $450 for a 40amp unit which leaves me with an unused 20amp device.

GerryP · Veteran Member

G'day Hylife,

Unfortunately, I'm not sure you'll get too many responses from anyone who has done this, and if you do, the chances are they'll have different chargers to you anyway.

Google does offer some advice on this (I just googled "DC chargers connected in parallel") and it seems to depend entirely on the "smarts" your charger has - the more smarts, the more likely they are to conflict with each other. However, it also seems to be the consensus that during "Bulk" charge mode most of the smarts are inactive and you should get close to double the input initially. It's only when the charger(s) start to back off that conflicts and/or hunting may occur.

I know the above probably doesn't you help much, but good luck with it mate. Let us know how you get on if you decide to do it.

Cheers

Peter_n_Margaret · Guru

If you have a "smart alternator", that is much more likely to be a problem.

Cheers,
Peter

nomadz · Veteran Member

G'day Hylife,

My first post on this forum!

I can't give you an answer about having done it but the following may assist!

I have a Projecta DC20 and am about to do exactly what you have asked about. I contacted Projecta's technical staff and they confirmed that what I want to do will work without any problems and that I will get up to 40 amps output from the two combined units.

They did, however, advise that I could not mix my (now superseded non solar) DC20 with the newer mppt enabled model. This was not a problem as I was able to pick up a superseded model at a good price.

The other thing they mentioned was to ensure that my cabling from the vehicle battery was adequate as the combined units could draw up to 57 amps. This is not a problem for me.

I will be having my 2nd DC20 installed in about 5 weeks.

Regards,

Ian

mr glassies · Guru

Why not buy a 40 amp in the first place

Dibs

Hylife · Guru

mr glassies wrote:
Why not buy a 40 amp in the first place

Dibs

Adding additional battery/s requires additional charging current, and as I stated earlier

Hylife wrote:
As lower powered devices are available in a bigger range of brands, are more competitively priced and are disproportionately much cheaper than higher powered ones, my thoughts were to purchase another identical 20amp DC-DC charger (approx $160) to the one I already have and to run two 20amp units in parallel, rather than fork out $450 for a 40amp unit which leaves me with an unused 20amp device.

One can purchase an additional 20amp unit for around $140 but a 40 amp unit costs well in excess of $450. Thats almost double the price and I am then left with an unused 20A device.

dorian · Guru

If you have two "dumb" chargers connected in parallel, then one will be doing most, maybe even all, of the work.

If you have two "intelligent" chargers connected in parallel, then one or both will probably shut down. This is because these devices monitor the output voltage and try to maintain it at their own preset level. For example, if one charger is trying to charge the battery to 13.8V and the other is set for 13.85V, then the second charger will be doing all the work and the first charger will probably shut down.

And no, I haven't tried this, just as I haven't tried many things which I believe to be potentially damaging or unsafe.

Aus-Kiwi · Guru

If you had two chargers and ONE regulator ? Then yes . But you'd have to know what your doing . I have moved battery to boot on race car . We found battery voltage a little low even with large cables . So we fitted an external regulstor close to battery . Problem solved . Atleast 2 DC and be parrelelled ..

03_Troopy · Guru

Why not contact the manufacturer of the charger you have, and ask them if it can be done.

Hylife · Guru

I anticipated that should the battery "set" only require lets say 10 amps input that one unit might do most of the grunt work, but what if the 200Ah set was at 50%? Cracking in 40amps (20 x 2) when my Toyota alternater is rated at 120amps should result in a much quicker recharge.

Heck for that matter, starting up the car and idling for an hour is not only going to chuck out more juice for the set compared to a generator, but it is cheaper on fuel and way quieter as well.

Because of a little fore-knowledge, my question was in anticipation of the current unit I have coming on special today (6th Apr) for $110.00

I've just bought a second one so I'll let you know if it works.

dorian · Guru

I did some reading on smart chargers and can see several potential problems when parallelling them.

https://www.batterystuff.com/blog/3-stages-of-smart-chargers.html
http://www.trojanbattery.com/pdf/U.S.%20Battery%20Charge%20Profile%20Full%20%2011-12-13.pdf

AIUI there are actually 4 stages (including the final trickle charge phase). Let's assume that the battery is a 12V, 200Ah AGM (C20 rating).

Stage 1 - Bulk Charge

- Constant current at 10% of C20 rating
- 20A (10% of 200)
- voltage can vary up to 14.4V (2.4V per cell)
- approximately 80% of total charge capacity

Stage 2 - Absorption Charge

- Constant voltage
- 14.4V (2.4V per cell)
- current can vary up to 3% of C20 rating (up to 6A)

Stage 3 - Finish Charge

- Constant current at 3% of C20 rating
- 6A
- voltage can vary up to 14.7V (2.45V per cell)

Float Charge

- Constant voltage
- 13.4V (2.23V per cell)
- unlimited time

Equalization Charge

- Constant voltage
- 14.7V (2.45V per cell)
- 1-3 hours
- repeat every 30 days

I believe that it should be OK to parallel two chargers during constant current phases. This is because each will be operating as a current source. The problems arise when the chargers switch to constant voltage mode. That's when the charger behaves like a voltage regulator. Each charger will be competing with the other for control of the battery voltage. AISI, the most likely outcome is that the one with the higher output voltage will take control and the other charger will shut down.

Another problem may arise when the charger attempts to measure the internal resistance of the battery.

Absorption StageSmart chargers will detect voltage and resistance from the battery prior to charging. After reading the battery the charger determines which stage to properly charge at.

In order to do this, I believe that the charger would first need to measure the open-circuit voltage (Voc) of the battery. It cannot do this if the other charger is pumping current into the battery at the same time.

Next it would need to draw a sample current (I) from the battery and measure the voltage (V). The internal resistance (R) would then be ...

R = (Voc - V) / I

Once again, the second charger must remain quiet while the first charger is testing the resistance.

nomadz · Veteran Member

G'day all,

I have now had the 2nd Projecta DC20 connected up and today was seeing 52 amps going into the 400 amp lifepo4 caravan batteries.

The extra 12 amps was coming from the solar panels.

Early days as only fitted yesterday.

Cable from start battery in tow vehicle to dc/dc chargers is 6 b&s and same from dc/dc's to batteries.

Regards,

Ian

Grahpet · Newbie

Could you run a second from the output of the first battery which is being charged by a dc-dc charger then you would be boosting the power of the first battery to charge the second. Not in parallel.

PeterD · Guru

GerryP wrote:
1.. I haven't actually tried two DC-DC chargers together, but it would be similar to having say a DC (or even AC) charger together with a solar controller, both on the same battery. You will get some additional benefit, but you most likely will not double your charge rate.
2.. What usually happens is that instead of seeing the battery voltage, they will see the other and one will probably decide that to 'back off' as it thinks the battery is at a higher voltage than it actually would be without the other charger connected.

1.. Agreed. I have not heard of any problems where two dissimilar chargers are used (say 240 V and solar.) None of them actually test the state of charge of a battery. They all monitor the battery terminal volts and are set up to react to the terminal voltage and switch to a float voltage when the charge current reduces to a low value.

2.. That is what you want to happen. When the battery has a low SOC the terminal voltage will be low. At that stage both chargers will supply what the battery will take out of them. When the battery terminal voltage rises to the absorption set voltage the charger with the lower set voltage will switch to its float voltage. That is OK because by that time the battery will have received its bulk charge and one charger is sufficient to finish the job.