Redarc BCDC Chargers 1225 vs 1240?

[Bmonck]

Morning all
I am looking at the battery charger/solar MPPT system at the heart of the electrics in truck.
I will be installing solar (around 175W) and have the normal user units (fridge, water pump, LEDs lights and charging points) but not heavy usage today.
I am currently deciding on the aux battery set up - either a c. 120Ah lithium or 2* 100Ah AGMs.
My goal is to be able to park up for e.g. a week without worrying about the batteries.

So I am looking at the Redarc BCDC units; the 1225 charges at 25 amps and the 1240 charges at 40 amps. The prices are not so different.
The redarc website says use the 1225 if you have an aux set up less than 200Ah and 1240 if above that. But reading around and even talking to Redarc customer support they say that the 1240 will be fine if you have more than one AGM or if you have a Lithium becos in both cases there is sufficient capacity of the battery/batteries to effectively take the 40amp charge. (As you prob know both Redarc systems checks the state of the battery each 100 seconds for voltage and adjusts its charge rate appropriately.) My concern is about not wanting to over charge the batteries.
A part of me says pick the 1240 as it allows for upgrade in usage/batteries in the future. Another part of me says keep it simple dont overbuild.
Anyway would really appreciate any real world experience or advice from all of you.
cheers

 

[Rosy]

Not sure about AGMs but Lithium batteries have a huge absorption capacity which should be stated on the spec sheet as shown in the sample below. Your limiting feature is likely to be the charge controller rather than the battery! Keep in mind your lithium battery will include an internal management system which will protect it from overcharging. If your going with lithium I suggest you get the beefiest charge controller you can to take full advantage of their absorption capability.

Im sure you've done the maths but 100A for a week seems low for anything that ideal "British summer" conditions. During our summer tour to the south of France, with ambient temp in the high 30s we were using upto 50A a day when the truck was parked up. The benefit of having lithium meant that as long as we did an hour of driving each day before setting up camp we had plenty of juice for the evening.

hope that helps
Rosy

 

[Bmonck]

Thanks Rosy
So this is my calculation of Amps generated by my solar panel (assumed 200w) generating around 13 amps per hour and assuming 4 hours of sun this creates a total amp generation per day of about 52 Amps/day.
My calculated demand is around 60 amps/day.
So unless my maths is badly off (which is possible) i am a net user of around 10 amps/day.
Assuming i start with fully charged 100Ah Lithium battery then i last 7 days (=net loss 70amps from Batt) until i need to think about supplementing the Solar with the alternator.
Does that make sense?

 

[uHu]

It makes something very near perfect sense Just dont mix up amps and Ah. If your solar panel can sustain 13 A continuously, and you charge for 4 hours, you get 52 Ah. That is theoretically tho', as there is some loss. Some of the power "boils" away as heat, and the chemical process is not 100% efficient. You should calculate about 20 % loss for charging and discharging (use) together. So, you will be "lacking" about 18 Ah per day in your example.

About the size of the charge controller: it is generally not possible to have a too big controller, as the battery will not take more amps than it can use, as long as the charger keeps to nominal voltages. So, the bigger the better, if there is any chance you may be able to use it.

Also, for battery size vs battery technology, check out the max discharge limit for Lithium vs AGM vs normal Lead Acid vs Silver Calcium Lead. If you then calculate in life expectancy, you may find that Lithium is the only way forward, giving you more usable capacity per £ per year.

 

[Rosy]

what @uHu said.... given all the variables that affect panel efficiency its best to be conservative with your estimates.

An alternative strategy for you to consider: If most of your week long stops are going to be at campsites with power then I would seriously consider a charge controller that supports 230V input which is exactly what I did. No need to worry about mounting, storing and adjusting panels as the light changes or worry about them going walkies if you need to leave your rig unattended. For me this provides the ideal compromise, as long as I can drive 1hr a day or pick a site that has power my beers will always be frosty.

Rosy

 

[DH.120]

Interesting article on the redarc site: When bigger is not always better

 

[Bmonck]

It makes something very near perfect sense Just dont mix up amps and Ah. If your solar panel can sustain 13 A continuously, and you charge for 4 hours, you get 52 Ah. That is theoretically tho', as there is some loss. Some of the power "boils" away as heat, and the chemical process is not 100% efficient. You should calculate about 20 % loss for charging and discharging (use) together. So, you will be "lacking" about 18 Ah per day in your example.

About the size of the charge controller: it is generally not possible to have a too big controller, as the battery will not take more amps than it can use, as long as the charger keeps to nominal voltages. So, the bigger the better, if there is any chance you may be able to use it.

Also, for battery size vs battery technology, check out the max discharge limit for Lithium vs AGM vs normal Lead Acid vs Silver Calcium Lead. If you then calculate in life expectancy, you may find that Lithium is the only way forward, giving you more usable capacity per £ per year.
[/

@ uHu. on my spreadsheet I am working on 200w of solar and assuming 20% inefficiency. Plus I am assuming a further 10% loss in terms of power to the battery.
on the Lithium controller battery I am on a steep learning curve. Some people have told me that even with the battery management system built into the lithium batteries it is important not to try to charge at too high a level. two different shops told me charging at 40 amps (from the Redarc 1240] might be too high. They talked about many batteries only being able to take 20% or 25% of their total capacity as a charge. So if it was a 100Ah lithium battery then the max charge rate would be 20 amps or 25 amps. This would mean the 1225 would be better than the 1240 and would prolong the life of the lithium battery.
I understand that of course some lithium batteries can take a higher charge rate.
what I don’t understand is if the 1240 is trying to charge at 40 amps and if the battery‘s own BMS limits this to say 25 amps what happens to the excess amps? Does it lead to excess heat or other probs ? (Sorry if this is a basic question but electrics are not my strong point .)

 

[Bmonck]

@Rosy - yes understood. We will sometimes be at campsites with shore power. In this scenario I am going to have a mains battery charger to keep the aux battery topped up. I will not run this through the Redarc, which can’t take a mains power input. Without spending significantly more I can’t find a bcdc charger that take mains input too. But please let me know if you have ideas. So parallel mains battery charger seems to work fine.

 

[Bmonck]

Interesting article on the redarc site: When bigger is not always better

@DH.FJ thanks good article. And it shows the there Redarc themselves recommend the 1225 for an aux battery bank less than 200Ah.
I did talk to a Redarc customer service guy about this. He said that a 1240 ie 40 amp charge to a 100Ah battery would be ok.
but as I mention above I-have talked to some other folks who caution against that. That is these other folks say better to go with the smaller 25amp charging from the 1225.

so all in all, just when I had talked myself into getting the 1240 I actually now believe the right decision is the 1225.
Happy rethink it again if people smarter than me on electrics have other views.
cheers

 

[DH.120]

Bmonck. Yes I agree, been doing the research myself as currently have a simple relay but want to move over to the redarc to allow for better battery management / solar charging and the option of changing to a better battery in the future.
The fact that redarc are not trying to flog their higher priced versions says a lot to me, and can see no reason not to believe them. Think the 1225 will be my Christmas present to myself

 

[Bmonck]

One nice feature on the redarc is the solar priority - so it takes power from solar panels first and minimises demand on the alternator.