varta powercaps hvc series...increase of internal resistance may happen. for recovery of the...

Application note

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VARTA Microbattery GmbH Daimlerstraße 1 73479 Ellwangen Germany

Release: 22.10.2015

Version: 02

Application note for NVDIMM backup with HVC 90F

Intermittent charging and SOH Check

VARTA Powercaps – HVC Series

High Temperature & High Rate Solutions

for Advanced Server Designs

Contents:

� Flowchart logic

� Recommended Charging Method

� SOH Check

� Typical Application Profile

(Charging Strategy & Backup Case)

Application note

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Release: 22.10.2015

Version: 02

This version of “Application note for NVDIMM backup with HVC 90F” contains an up-

date on intermittent charging. It describes how to enable fast charge for the HVC

90F.

Nevertheless, all information about intermittent charging with current limiting resistor

in version 1 is still valid.

Application note

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Release: 22.10.2015

Version: 02

The Powercaps HVC 90F series is a real hybrid system combining electrostatic and

faradaic energy storage. It has more power density than a battery and more energy

density in than an ultracapacitor. Therefore, faster charging than a battery is possible

(see figure 7 and 8).

For applications requiring a constantly high charge state of the energy storage de-

vice, e.g. backup systems, we recommend an intermittent charging method. The idea

of intermittent (/pulsed) charging is to compensate self-discharge while avoiding

regular overcharge in order to improve the lifetime of the Powercaps system.

The recommended and simplest way to implement intermittent charging is a constant

voltage source controlled by a timer.

During operation the state of health (SOH) of the energy storage device has to be

monitored. An initial charge ensures enough energy for the next application while

maintenance charge pulses compensate self-discharge, see also section 1.6.

Typical steps:

1) Secure available energy by OCV (open circuit voltage) check and initial charg-

ing

2) After initiation regular operation begins – maintain charge state, monitor state

of health (SOH)

3) Discharge scenario (backup) � 1)

A flowchart logic for operation is illustrated in figure 1. The initial charge in a typical

application is demonstrated in figure 5 and 6.

Application note

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Release: 22.10.2015

Version: 02

1 Flowchart logic

Intermittent charging routine

Fig. 1: Flowchart logic for operation of backup system with intermittent charging (example)

Server running

SOH Check 1 A dummy load for 100 ms Measure voltage under load (CCV) Calculate ESR(DC) Set SOH timer to 0

yes

OCV < 1.29 V / cell

Initial Charge ON Time: 5 - 10 min Set charge timer to 0

no

Maintenance Charge ON Time: 1 min Set maintenance charge timer to 0

no

Power ON

Powercaps present?

SOH timer

> 2 d

Maintenance charge timer

> 12 h

no

yes

yes

Application note

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Release: 22.10.2015

Version: 02

1.1 Power source

The power source voltage (US) should be adjusted to the Powercaps configuration,

see table 1 and figure 2.

Fig. 2: Schematic circuit diagram

The charging voltage (UCV) has to be slightly higher than the rated voltage (UR) of the

respective Powercaps assembly, e.g. 4.35V for HVC 90F rated with 4.2V. This in-

creases the charge current. UCV in table 1 is the voltage under load directly measured

at the Powercaps pack. Always consider that there might be additional losses be-

tween the power source and the Powercaps. CV charging means that UCV is applied

in full at the Powercaps (under load).

Only if the temperature is above 40°C UCV should be reduced to UR.

Table 1: Typical values for intermittent fast charging

Parameter Setting

Constant Voltage

Setting

UR --> UCV

1.4 V --> 1.45 V

2.8 V --> 2.90 V

4.2 V --> 4.35 V

5.6 V --> 5.80 V

7.0 V --> 7.25 V

8.4 V --> 8.70 V

9.8 V --> 10.15 V

11.2V --> 11.60V

UCV Tolerance + / - 10 mV

initial charge 5 - 10 min

maintenance

pulse1 min

maintenance

pulse timer12 h

SOH Timer 2d

dummy load 1 A

Application note

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Release: 22.10.2015

Version: 02

Alternatively, when the source output cannot meet the required criteria, the residual

current which is present when reaching fully charged state has to be minimized. This

can be achieved by adjusting the “charging ON-time” to the residual charge current,

especially during the initial charge step, see examples in section 2. Again, the aim is

to minimize overcharge.

Operating Powercaps in a wide temperature range may require a temperature com-

pensation of the charging voltage UCV by a linear function (Please contact our appli-

cation engineers for more details).

Application note

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Release: 22.10.2015

Version: 02

1.2 Resistor

For the intermittent fast charge, a current limiting resistor is not necessary. Instead,

UCV at the Powercaps pack is slightly increased compared to intermittent charging

with current limiting resistor (described in application note version 01). This increases

the charge current. High peak currents above 2A might occur for milliseconds if the

Powercaps is completely discharged. However, because of the short overall charging

time, continuous overcharge is avoided.

1.3 Powercaps present?

After the device is turned on, it has to be checked if a Powercaps is present. A volt-

age below 0.8V per cell indicates a damaged or over-discharged pack.

1.4 OCV Check

The open circuit voltage has to be measured periodically. If the open circuit voltage is

below 1.29V per cell an initial charge has to be applied, see figure 1. Additionally

check if the maintenance charge timer is higher than 12 hours, see section 1.6.

One measurement per second is sufficient. More measurements could create an ad-

ditional leakage current which should be avoided.

1.5 Initial charge

Initial charge is necessary if the voltage per cell is lower than 1.29V per cell. This val-

ue is independent of the temperature. The “ON-time” of 5 - 10 minutes in the flow

chart is a typical range. It depends on the required energy of the application and will

recharge the Powercaps sufficiently for the next use case. If significantly less than

115J per cell are required, the “ON-time” can be reduced. Initial charge is most likely

necessary after a backup case, when the system is turned on for the first time, or af-

ter power was off for several days.

Application note

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Release: 22.10.2015

Version: 02

1.6 Maintenance charge

Maintenance charge is controlled by a timer. Short charge pulses (typically 1 minute)

will keep the Powercaps sufficiently charged and compensate self-discharge, see

example in figure 3. Through the very short overall charging time of less than 3

minutes per day the total amount of overcharge is minimized.

Fig. 3: HVC 90F maintenance charge 1 min every 12 hours at 45°C

In figure 3 a HVC 90F 4.2V is charged with maintenance charge for 25 days (at

45°C). The maintenance charge pulses keep the Powercaps fully charged and com-

pensate self-discharge.

A recharge of 10mAh per day is sufficient and will not age the Powercaps through

overcharge. Additional leakage currents might have to be considered.

Application note

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Release: 22.10.2015

Version: 02

1.7 ESR and expected lifetime

During service life a gradual increase of ESR(DC) can be observed. This increase is

nonlinear over time. The end of life (EOL) is reached, when the equivalent series re-

sistance (ESR(DC)) is too high to satisfy the required power demand.

This slight increase accelerates after approx. 3/4 of the lifetime which can be used as

an early indication for EOL, see figure 4.

The exact lifetime depends on the power requirement, charging condition and tem-

perature profile of the application. Five times the original value is a typical factor for

the ESR(DC). Any additional resistance, e.g. through cables and connections has to

be considered, the factor five is related to the cell resistance exclusively.

Fig. 4: Typical change of ESR

According to R = U/I the ESR(DC) value can be calculated based on voltage drops due

to short load pulses, see also 1.9 SOH check.

Application note

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Release: 22.10.2015

Version: 02

1.8 Temperature dependency of the ESR(DC)

The ESR(DC) depends on the temperature, see figure 5.

Fig. 5: Correlation between ESR(DC) and temperature

Only values measured in the same temperature range should be compared. Range is

+/- 3°C for temperatures below 30°C and +/- 5°C for temperatures above 30°C.

It is also possible to measure the ESR(DC) at a certain temperature and calculate the

resistance at a reference point, e.g. 20°C. Therefore, it is important to measure the

temperature near the Powercaps.

Application note

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Release: 22.10.2015

Version: 02

1.9 SOH Check

The state of health (SOH) of the Powercaps can be determined by monitoring the

increase of the ESR(DC) in the application. The ESR(DC) can be determined by apply-

ing a short dummy load of e.g. 1A to the system. A load close to the discharge cur-

rent during use case is recommended, but not lower than 500mA.

Just before the dummy load is applied an OCV voltage measurement is conducted

and compared to the voltage under load after 100ms (50 – 250ms), see figure 6.

Fig. 6: HVC 90F 2.8V 1 A dummy load for 500ms

According to R = U/I the ESR(DC) value can be calculated from the voltage drop ∆U

due to the dummy load.

The SOH check should be done when the Powercaps pack is in equilibrium. This

means when the OCV voltage is stable. After charging and discharging, the OCV

voltage will drift to a stable level. This can take some minutes, depending on the

temperature which affects the kinetics of the electrochemistry. Therefore, the SOH

check could be done right before the maintenance charge pulse, see figure 1

(flowchart). This ensures 12 hours between SOH check and the last charge pulse.

Application note

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Release: 22.10.2015

Version: 02

1.10 Storage

Ongoing deep discharge during storage has to be avoided, i.e. OCV voltages below

1.0 - 1.1V per cell.

For short periods of time, e.g. during a backup, the voltage under load (CCV) is not

critical. As soon as the load is disconnected, the OCV voltage will drift towards a sta-

ble level above 1V per cell. This happens due to the hybrid character of the

Powercaps, see OCV relaxation in figure 7.

However, prolonged periods of time in deep discharge condition will cause deteriora-

tion of performance characteristics. After prolonged storage without any recharge an

increase of internal resistance may happen. For recovery of the performance a re-

fresh cycle is recommended.

Therefore, it is important to minimize leakage current during storage and to recharge

after a backup. The Powercaps should be protected by a MOSFET in order to have a

very small leakage current caused by the circuits of the device (max. 5µA – 15µA,

recommended in nanoA).

Application note

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Release: 22.10.2015

Version: 02

2 Typical backup scenario

Example 1

Fig. 7: Typical application profile for a HVC 90F 4.2V at 40°C and a 3W backup load profile.

The HVC 90F 4.2V Powercaps device in the application example operates at 40°C,

which is typical for a NVDIMM backup operation (figure 7). The fresh Powercaps de-

vice was stored for four months at room temperature. Due to the low self-discharge it

can be charged within one minute. After a pause of 10 minutes a discharge with 3W

for 285 seconds is possible until the lower cut-off voltage of 2.4V is reached. This

corresponds to an available energy of 850J for a 4.2V pack (in this example). A typi-

cal backup case for DDR3 8GB RAM requires only about 360J at 3W for 120 s.

Hence, for typical operating conditions two backups in row are possible.

Charge 1 min

Rest

(OCV relaxation)

3W Backup Rest

(OCV relaxation)

Application note

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Release: 22.10.2015

Version: 02

Example 2

Fig. 4: HVC 90F 4.2V SOC0% at 40°C, initial charge and backup case

As a second example, a completely discharged HVC 90F 4.2V Powercaps device

gets recharged by an initial charge within 10 minutes terminated at a cell voltage of

4.34V, see figure 8.

This voltage threshold (4.34V) is adjusted to the residual charge current of 400mA,

caused by a 4.8V power source (R= 1.2Ohm). This additional criteria for charge ter-

mination is not required if the voltage of the power source is set to the values given in

table 1. In this case a simple timer is sufficient.

After a relaxation phase of 10 minutes, a discharge (backup) time of 283 seconds is

available for a 3W load until the cut-off voltage of 2.4V is reached, which corresponds

to an available energy of 850J. The ambient temperature in this example is 40°C.

During regular server operation the Powercaps would not be completely depleted,

except after several years of storage. Thus, under typical application conditions re-

charge after a backup or longer server shutdown periods is faster. In general, the

recharge time depends on the state of charge and the required energy of the applica-

tion.

3W Backup Initial charge

Rest

(OCV relaxation)

Application note

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Release: 22.10.2015

Version: 02

3 Summary

Intermittent charging by a simple timer controlled CV method is recommended for

best performance and long service life of HVC Powercaps.

The aim is to maintain a high state of charge (SOC), to be ready for a backup case,

while minimizing unnecessary overcharge. After a backup scenario Powercaps can

be recharged quickly due to the innovative hybrid cell chemistry.

During operation the state of health (SOH) can be monitored by short (ms) meas-

urements of the ESR(DC).

Powercaps are the ideal solution when more energy is required for reliable backup

applications.

varta powercaps hvc series...increase of internal resistance may happen. for recovery of the...

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